First period, Monday morning, 28 S4 pupils sit before you, some bright eyed and eager, others yawning and stretching. Look around carefully. Within a year one of those pupils will be homeless.
This scenario is, of course, not literally true, but over the past year 4,339 16- and 17-year-olds in Scotland (that is one in 28 of the year group) presented to their local authority because they had nowhere safe and secure to stay.
So, while few S4 pupils think homelessness will affect them personally, the figures tell a different story.
A new campaign to raise awareness about homelessness among both teachers and teenagers has been launched by the Scottish Council for Single Homeless to coincide with the 40th anniversary of the broadcasting of the TV play Cathy Come Home on December 16.
While the anniversary is being marked by a week of BBC programming on homelessness, the SCSH hopes that the media focus will not only promote topical debate in public and political arenas, but that secondary schools will take the opportunity to raise the matter with every S4 pupil.
To this end, the organisation has sent each Scottish secondary a Home Smart teachers' pack, with facts about homelessness and advice on how to avoid it. The pack contains the resources for a 45-minute session, with extras for those who have time to do more.
Part of the thinking behind the Home Smart campaign came from a youth tenancy conference the SCSH organised in Dundee two years ago where, with the benefit of hindsight, young people argued that independent living skills, such as finding accommodation, dealing with isolation, managing your door, budgeting, paying bills and dealing with debt, should be part of the school curriculum.
"It's about helping every S4 pupil to understand that homelessness is something that can happen to anyone, ensuring they know where to go for help and advice and encouraging them to appreciate the challenges of leaving home at an early age and the difficulties it can lead to," says Mark Elton, head of the SCSH Youth Unit.
"Sixteen-year-olds often have little idea of what life outside the family is like and if they don't know where to go for help, especially if they are cut off from their families for whatever reason, problems can spiral quickly when they leave home," says Dr Elton.
If the knowledge base of 16-year-olds is shaky, with 15-year-olds it is "extremely low".
Dr Elton describes a school session where, at the start, the majority of 15-year-old girls said they would probably leave home around the age of 19 or 20, the boys around 22 or 23. By the time Dr Elton had led a discussion through private and social housing, rent, council tax, waiting lists, utilities and telephone bills, TV licence, food, travel, clothes and then the costs of "having a life" - nights out, take-away meals, CDs, DVDs, holidays - the desired leaving age rose by three or four years for both gender groups.
"Some joked they didn't actually want to leave home at all," says Dr Elton.
"But the main message was clear. They simply didn't understand the range of things they have to manage to run a home, and that's why targeting schools is so important.
The teachers' pack fits well into the Curriculum for Excellence and is useful for personal and social education, religious and moral education or English, he says.
"It's about creating able citizens. It raises awareness about an important social issue and it promotes financial literacy and the understanding of civic structures while giving practical advice."
Any doubt that such advice is needed is perhaps best answered by the pupil who, on learning about the work of Shelter - which is also marking its 40th anniversary this year - said: "Oh, I thought it was just a charity shop."
Scottish Council for Single Homeless, information on the Home Smart campaign and schools materials, www.leavinghome.infohomesmartAll schools are being encouraged to register leaving home and homelessness education activities with the SCSH by the end of December and could win up to pound;200 worth of books or sports equipmentShelter's free housing advice line, tel 0808 800 4444http:scotland.shelter.org.uk