Take a typical classroom: the children have to be there by law; the teacher has to be there; the teaching assistants also have little choice.
But at the back sits someone who does not have to be there - a mother with young children in nursery, a grandfather who has retired, a local office worker popping in for an hour or an undergraduate looking for work experience. Not only do they not have to be in your classroom, they are not being paid. They are volunteers, just a handful of the thousands who lend a hand in schools around the country.
Some 25 - including a former BBC correspondent - have been helping out at St Mary's Catholic Primary in Battersea, south London, giving 75 children an hour's extra reading practice over the year. In July, the school announced that it had seen a dramatic rise in key stage 2 results: 91 per cent of pupils had achieved the expected level 4 in both English and maths, compared with 52 per cent in 2011.
The success was a front page story for the London Evening Standard newspaper, which has "adopted" the 160-pupil school, helping to secure a pound;100,000 donation from Ukrainian billionaire Kostyantin Zhevago to fund dozens of helpers from the charity Volunteer Reading Help (VRH).
But Jared Brading, executive head of St Mary's, and Sue Porto, chief executive of VRH, both stress that using volunteers does not guarantee great test scores. They know that the key to getting the most out of volunteers is building relationships within schools that neither overplay nor devalue the role of people who give up their time for free.
"If you want volunteers to have a meaningful relationship, you have to welcome them, make them feel valued. We have a tea party for them at the end of the year, to say thank you," Mr Brading says. "The reading volunteers have added an enormous amount to the life of the school. But the Sats results are primarily down to the hard work of teachers and children.
"The volunteers have supplemented and complemented that, and in a school where most children have English as an additional language, the opportunity to read with a trained adult is great."
Ms Porto adds: "The most important thing in terms of relationships, and the thing we stress with schools, is that we're not there to replace teachers. We are there to help provide additional support that some kids need."
Use them wisely
So what do you do when someone turns up in your classroom offering to help?
"If people are volunteering, you want their time to be used well," says Sara Bubb, a senior lecturer at the University of London's Institute of Education. "Just because they are not getting paid is not to say you don't need to think about how to deploy them well. As a mum volunteer myself, I've really liked it when people know my skills and deploy me sensibly.
"I'm a teacher, so as a volunteer I could do anything, including taking your class, but sometimes I've gone in and would be sewing. I just felt doing cross-stitch was a bit of a missed opportunity. Why not give me something more challenging to do? It is lovely to have teachers in your class as volunteers because you know they can do anything."
Ms Bubb adds: "Volunteers are a fantastic resource. You need to find out what they are good at and play to their strengths. You don't want them bored senseless or terrified."
And, of course, people are unlikely to simply turn up. Many schools now have someone, often the assistant or deputy head, who is responsible for coordinating volunteers. The first job, Ms Bubb suggests, is not to grab anyone who is offering their time and give them a task that needs doing, but to find out what skills people have to offer that the school may be able to use.
Schools may often ask for parents who can help with certain tasks, but another approach suggested by Professor John Bastiani in his report Involving Parents, Raising Achievement is a semi-formal inventory that lists parents' cultural knowledge, work experience and special interests.
So someone does not just turn up in your class vaguely offering to help, but arrives knowing they have something to offer, whether it is reading, art, gardening or music. But there is more to teaching than being able to do something yourself. Helping volunteers communicate well with children is a key part of a teacher's role.
"One thing to bear in mind is that volunteers will model themselves on the teacher - on how the teacher speaks to children and how they manage behaviour," says Ms Bubb. "There are a lot of unspoken things that will be picked up by volunteers about the culture in class."
And another tip - do not put parent volunteers in the same class as their own children, because their relationship outside school can distract from the task at hand.
At Water Hall Primary in Milton Keynes, where 43 per cent of children are on free school meals, there has been a noticeable change in the type of volunteers. Increasingly, people are looking not just to share their experience, but also to gain skills. This means that while the primary concern of a school is the needs of the children, a successful relationship will also take into account the needs of volunteers.
Deputy head Karen Roberts introduced a formal policy a couple of years ago to cover the school's work with volunteers. Prospective volunteers first go for an informal chat with her and, if they want to go ahead, Criminal Records Bureau or Disclosure Scotland checks are done. Once all the paperwork is complete, a more formal interview takes place, where volunteers are told about the school's policies and training is given, if required.
"What we don't do is just say, `Yes, there you go,'" says Ms Roberts. "Because we're about raising standards so that volunteers can be really helpful and useful and have a lot of skills to offer. But we need to make sure they are moving children's learning on.
"If they're reading with children, we explain that we encourage children to turn the pages themselves and point to the words themselves. It's worth investing a little bit of time with them."
While the school is keen to ensure volunteers have training where needed, it also has different sorts of volunteers - parents, secondary school pupils, undergraduates - doing jobs ranging from helping with literacy to accompanying school trips, working in the library and helping to serve free breakfasts.
And as well as the benefits for pupils and the experience gained by the volunteers, Ms Roberts points out that welcoming people into school helps children in a less tangible but still valuable way by binding the community together.
"It makes people feel they can do things," she says. "We work really hard to get parents across the door - a lot of parents see schools as frightening places because they didn't have a good experience of school.
"We want to break down those barriers and we never want to turn away a volunteer. We find something they are capable of doing because they are part of the community and we have a duty to support them in that way. And, at the end of the day, that will help children."
Becky McGuire, a teacher at Water Hall, enjoys working with volunteers. "The school wouldn't fall apart if we didn't have them," she says. "However, they bring lots of different qualities to children's learning. When our topic was India, one Indian parent came in and cooked Indian food with the children. Another time we had some Bengali dancing.
"I remember one reading volunteer when I was (working) in Year 5 who was just brilliant at listening to children read. She would ask the right questions and just had that knowledge of how to talk to children. I really saw a difference with those children who had that extra help."
Ms McGuire advises meeting volunteers before they come into the classroom, to talk through what they want and what the teacher wants - even putting a plan in writing, if appropriate, so that they know which children they will be working with. This is also the time to sort out small but important details, such as whether they prefer the children to call them MissSir or by their first name.
Bastiani, J. Materials for Schools: involving parents, raising achievement (2003). Former Department for Education and Skills. dera.ioe.ac.uk9814
HOW TO MAKE USE OF VOLUNTEERS
- Make one person responsible for volunteers.
- Consider drawing up a volunteer policy to cover aspects of work such as Criminal Records Bureau or Disclosure Scotland checks, whether training is required, rules on child confidentiality and whether parents work with their own children.
- Make volunteers feel welcome.
- Find out what skills volunteers have (consider making an inventory). People do not want to feel underused, but make sure they feel comfortable with what you are asking.
- In the classroom, remember that the volunteers will copy how you behave with the children.
- Be very clear about what you expect a volunteer to do - put it in writing if necessary. Do not waste their time or yours.
- Make time for chats about how things are going. If volunteers are working with individual children, have some way they can feed back to you.
- Invite volunteers to school assemblies and performances. They are part of the school community.
- Show your gratitude and celebrate their contribution.
- In three words: welcome, communicate, celebrate.
DOING YOUR BIT - BIT BY BIT
Micro-volunteering is giving small amounts of time on a one-off basis.
Orange's Do Some Good smartphone app is one way of taking part. The app works by giving people opportunities to volunteer for four hours in their community, in return for music rewards.
A survey of 3,600 people who volunteered this way, by the Institute for Volunteering Research, found that half did not care about the reward and a fifth did not even know about it.
The survey found that this way of volunteering seemed to attract young people, who were less likely to engage in volunteering action and charitable giving.
They seemed to be driven by a "why not?" attitude. They wanted to fill some spare time and participation depended on how convenient the activity was.