My tradition holds courses for children (8-12) and teenagers (13-17) from a one-day course on anapana meditation to seven-day vipassana courses for teenagers. Visit http://www.vri.dhamma.org
for more info. Two of my kids have done the children's course but my older son isn't interested in attending a teenager's course - and I'm not sure what the course content is like. The kids courses are very relaxed and the focus is on activities with four fifteen minute sessions of anapana thrown in - and a fairly light discourse at the end of the day.
For a few years I have been invited to speak to our year 10 (16-year olds) students on the subject of 'the Buddhist perspective on death and dying. Its during a program where other religions are introduced by looking at how they approach death and dying. So, that could be one subject one could introduce.
If you have a Buddhist Society in the UK you may wish to contact them and see if they can offer any assistance by way of program outline and resources.
All the best.