My sister did A level and from what I saw I have this to say:

I would get only a few DSLRs and focus on making sure you have a decent supply of digi cams for the students. Most will have digi cams of thier own, but have the option of hte school kit there for those that don't (or have weaker models).
As for the DSLRs I would get some of hte cheaper models (a 400 or even the 350) and focus after that on getting decent lense to go with them (the lenses being more important at this level than the camera body).
After that I would teach them to use the DSLR and how it works as they will give a better understanding of photography than the digicams will - but don't force them down the using DSLRs at this level (leave it for the student to have the interest and time to have the interest in the DSLR). I say this because at the end of the day they are out for a good grade and (especially for those newer to the course) chances are that they can get better shots with simpler kit than they can with more expensive kit.
(though I accept that "good shots" are not always the best and an artistic skill can be shown in technically poor shots, but the students also have ot have confidence in themselves and some good clear sharp digicam shots can do wonders for this