Teachers may cringe when they are referred to as "lifelong learners" but that, in effect, is what they are. The best never stop extending their skills, curiosity and understanding.
This is particularly true for science teachers. They need to keep abreast of fast-moving developments, change and research, especially if they are teaching out of their comfort zone.
Someone with a degree in biology may well be asked to teach physics and chemistry as well - shortage subjects that often lack specialist practitioners. In some cases, A-level science pupils will be better qualified to tackle the curriculum than their teachers, making further training essential.
"Science is different from other subjects," says Professor Mary Ratcliffe, associate director at the National Science Learning Centre (NSLC), which provides continuing professional development (CPD) programmes.
"Many teachers won't necessarily feel all that confident teaching across all the sciences or conducting good, effective practicals. It's crucial they have access to the highest quality courses that allow them to up their game."
Although the courses are out there - including many that are paid for or heavily subsidised - schools can be reluctant to pay for cover.
The problem has intensified since September, when the "rarely cover" rule came into force. This means schools receive no extra money to hire supply teachers or cover supervisors.
So should a teacher go on a day's course, schools will either have to hire a supply teacher for up to #163;200 or a cover supervisor for about #163;7 an hour. The latter may be a cheaper option, but cover supervisors are not allowed to teach.
Many schools are responding by cutting back on school trips and external CPD courses. Simon Decker, headteacher of Rainham Mark Grammar School in Gillingham, Kent, believes the "rarely cover" stipulation will cost him an extra #163;35,000 this academic year. His chair of governors has already written to Ed Balls, the Schools Secretary, asking that the upper limit of 38 hours for cover per year is re-introduced.
"Rarely cover has removed the flexibility we used to enjoy," says Mr Decker. "We have to employ cover supervisors for a full day even if it's just to cover two or three lessons. It's hugely wasteful."
Mr Decker is already proposing fewer trips due to the huge additional costs, and is more prone to hold in-house training afternoons or Inset days for his staff. But he is adamant that external CPD days should continue.
"It's imperative that teachers have top-quality training," he says. "It reminds staff why they came into the profession in the first place and allows them to find out more about their subject. It has a direct impact on enthusiasm, standards and teaching and learning."
Rainham was one of 51 schools to be awarded the Prince's Teaching Institute's (PTI) first Schools Programme Mark in November, which recognises inspirational ideas and activities that enhance English, history or science teaching.
The award builds on the PTI's Cambridge University summer school for subject specific CPD. It is rooted in the belief that in-depth subject knowledge and enthusiasm are crucial to effective communication in the classroom.
A member of the biology department at Rainham School attended a PTI event that focused on the work of Darwin. She returned brimming with ideas and co-ordinated a range of events, including a party in February to mark Darwin's 200th birthday and the 150th anniversary of his seminal work, On the Origin of Species.
She also orchestrated hand-painted maps of the Galapagos Islands to be displayed in the corridors, plus pictures of members of staff as they might have looked thousands of years ago. Pupils had to guess who was who.
Ideally, the whole department would have such an opportunity, but Alison Goodall, head of biology at Rainham School, could only afford to send one person. Rarely cover makes it even harder. Still, she thinks it is imperative that schools find a way.
"The benefits are so much wider than enhancing a single teacher's performance," she says. "It can have a huge impact on the school, pupils and other teachers."
Ms Goodall herself went on one of the PTI's residential summer courses a few years ago and came back buzzing. "It's just so nice to step out of your everyday routine and reconnect with your subject and why you love it," she says.
"It renews your enthusiasm. The speakers were all recognisable and talked with such passion about education and their subject. It is rare to have such a strong subject focus."
Being treated as professionals was at the heart of a chemistry day last year for 30 local schools in Kent. Representatives from the perfume industry and GlaxoSmithKline (a pharmaceutical company) worked alongside teachers and experts from the Royal Society of Chemistry.
"I learnt a lot and had very positive feedback," says Chris Dixon, head of chemistry at Rainham School. "It helped us see the relevance of the subject in the real world and the job market, which we have passed on to our GCSE and A-level pupils."
Releasing members of staff to go on courses is a big commitment, says Andrew Linnell, headteacher of Desborough School in Maidenhead - another school that was awarded the recent PTI mark for science. But it is critical to raising standards.
"If it wasn't worth it, I wouldn't do it, because it does eat into our time and budget," he adds. "But we believe it recharges teachers' batteries. It reinvigorates staff, who then take that stimulus back into the classroom."
Staff from the school's biology department have worked closely with the Royal Holloway, University of London, which has offered teachers practical sessions on electrophoresis techniques.
Specialist scientific equipment, such as a PCR machine (a thermal cycler), was also donated to Desborough by the university. One of its researchers, Safina Khan, then visited the school to run demonstration lessons using the equipment.
"It gave our teachers a boost and a focus," says Mr Linnell. "It increases their portfolio of skills and their own competence."
University lecturers typically spend most of their week on research, pausing to teach for just a handful of hours, Mr Linnell adds.
In schools, it is the direct opposite: teachers get to plan and prepare for just a couple of hours a week and teach for the remainder. By taking time out to develop their subject knowledge, teachers' lessons are more likely to "sparkle", he believes.
Although the investment in external training is a "deliberate attempt to pamper" by Desborough School, it produces hard learning outcomes.
"There is some loss from having teachers out of the classroom but the benefits far outweigh the negatives," explains Mr Linnell.
"We have to find a way because lessons are routinely better as a result."
Ms Ratcliffe would like to see all headteachers place such a high value on CPD. The National Science Learning Centre runs a bursary system that covers course fees, but she recognises that the "rarely cover" rule could act as a deterrent.
"What we now have is an agreement that addresses teachers' working conditions but doesn't address how schools maintain and develop their workforce," she says.
"Staff need to be at the cutting edge of their subject. They have an entitlement to high-quality CPD."
She admits, however, that the training landscape is varied at best. Traditionally, schools have found it hard to identify training that is both appropriate and really effective.
Susan Anderson, head of science at St Marylebone CofE School in central London, has seen at first hand how poor staff training can be.
"If your one day a year had not been taken up with a course to find out about the new GCSE or A-level course, you were confined to a three-star hotel in central London with an 'expert' trainer who talked at you for five hours," she says.
Needless to say, a hotel conference room was no place to mess around with chemicals or try out new and exciting experiments. Now, Ms Anderson sends staff from her department on Project ENTHUSE courses - fully funded courses run by the NSLC.
The course leaders at the London Science Learning Centre, plus the academics at the University of York, are highly skilled and inspirational, says Ms Anderson. It also helps that the course labs are similar to the ones you would find in school - neither too plush nor too sparse.
"It sounds silly, but just going on a train up to York for the day is so important to morale," adds Ms Anderson. "You feel valued. It's nice to get out and have a conversation with other like minded people about your subject."
St Marylebone is very lucky to have a 15-strong faculty with specialists within each of the sciences. This allows senior members of staff to run in-house training after school or during Inset days that combines national agendas with information about new resources and ideas.
An added bonus is that those staff will be able to put the learning into context - with one eye on the school's unique environment, be it a high level of special needs children or pupils with English as an additional language.
It is therefore good to have a mix of training options, adds Ms Anderson. She recognises, though, that rarely cover may limit external training opportunities. The more money that goes into the school's cover budget, the less is available for CPD courses or travel expenses.
"There has to be some flexibility built in so that schools can run their cover as they see fit. It is too important not to," she says.
"Every member of department who has been on the NSLC course comes back enthused. They then pass that enthusiasm on to other members of the department."
This chain reaction is crucial to building up science teachers' skills, argues Ms Ratcliffe. But it can only work if teachers are released by their heads.
In theory, leaders want freshly enthused and knowledgeable staff. In practice, they also want healthy balance sheets.
If trips and external CPD courses are too costly in terms of cover, heads may feel they have no choice but to give them up for good