If everyone throws in their two cents, pretty soon you'll be rich.
As mentioned, the height. 7' is great if you have a vehicle that can pull it, knowing that the "wind drag" going down the highway is just as bad as the weight.
Length and number of axles go hand in hand, almost. The longer trailers almost always go tandem. Check the different manufactures.
Weight distribution is easier to accomplish with a tandem axle (you don't have to be as precise as you do with a single axle). A tandem axle is automatically adding weight.
So somethings to think about,
1. Do I really need it 7' high, or can I have the short students load it?
2. How much do I really need to travel with? (overall capacity)
3. Is there someone else with a larger vehicle that can pull a trailer?
Only you and your other mentors know what you really need, however this thread has provided a lot of important points to think about.