I think Marine Depot has the most competitive prices, plus look under the education tab. I just read a great article today. You could get all the kids to subscribe to Marinedepot's website by going to: MarineDepot.com
I also agree with the grant writing part. Grant writing does take time, however. If I ever get enough American Airlines miles saved up I'd happily come set up your aquarium.
Each student could be responsible for testing for something like Ammonium, Nitrites, Nitrates, Phosphorus, Calcium, pH, etc. In my college biology days we had to spend half hours describing the behavior of certain animals, and drawing them! Your students likewise could do this, and maybe present to the class what they have learned by observation about fish behavior, how the nitrogen cycle works, the purpose of live rock in a salt water aquarium, and why a 2 degree increase in the oceans temperature is killing our coral. Coral is something else that would well be worth mentioning because of the tiny polyps that live within the hard structure of the coral.
Each kid could create a scientific notebood and be taught about recording observations properly. This could light a fire for future marine biolgists.
I wish you so well as you endeavor to follow your dream. I am happy to give you whatever I am not using. Just post a list; maybe you don't know all that you need--just thought of that. Let me know.
Take care, and have a relaxing Sunday, cordially,
Matti