A DVD as you suggest might be useful. It would help get them up to speed on the basics. There are some good books that will provide some that sort of background too of course, but a properly done DVD might in fact help them out a lot.
One needs to keep in mind also that the goals and objectives for a classroom tank are not altogether the same as for a tank a hobbyist might set up at home. For example, the ultimate
SPS tank would probably not be the ideal classroom tank (though the ideal classroom tank SHOULD have adequate conditions for healthy growth of
SPS corals, in my opinion, but even tanks that will not support SPS corals can be very valuable in a classroom). These ideas were all discussed in some detail in my MACNA talk, which I hope to have an opportunity to give to TCMAS at some point in the not too distant future.
The DVD idea is a good one. However, I think it should also be said that what a teacher would need even more than a DVD is a knowledgeable person that they can rely on for help and good advice, both at the start, and on an ongoing basis (for a few years at least). I think that in most cases, a classroom reef tank will simply not happen without that.
Regarding tank size, a tank in the neighborhood of 20 gallons would certainly be more affordable, and it coud be very useful. Certainly lots better than no tank. It would also have the advantage of being small enough for a teacher to transport to his/her home in the summer if necessary.
A 20 gallon tank would be somewhat limiting though. If a school had the space, and came up with sufficient money or donations, I really think that something on the order of 40 gallons minimum, or preferably more like 65 gallons or larger would be much better.
Also, I would never use an acrylic tank in a classroom where multiple students will be taking care of it. The inside walls of the tank will be scratched in no time (and the outside of the tank might get scratched as well).
Thanks for your interest in this classroom aquarium idea.
(Thanks for your PM, by the way).