In general you can start off with most of the same equipment that you would use in a freshwater setup, (hang on back filter, heater, thermometer, etc). However you will probably want to use live rock or dry rock, this is the main biofilter in most saltwater tanks. A powerhead would be good to use as well to keep the water moving and oxygenated. A sandbed isn’t required but I prefer them over bare bottom tanks. A protein skimmer would be helpful but isn’t entirely necessary as long as you’re careful with stocking and feeding.
If you want corals a decent light is necessary, but there are affordable options if you plan on only keeping “easy” corals.
When you set up the tank you’ll have to let it cycle like with a freshwater tank, I’d recommend using a bottled bacteria product to speed up the cycle. I prefer instant oceans biospira.
As far as stocking goes, I’d start out with small hardy fish. Clownfish would probably be ideal but there are plenty of other hardy choices.
I’m not sure what your source of water will be, but assuming you will be making saltwater at school you’ll need an RODI system and some kind of container to mix the saltwater in. Brute trash cans are popular, you’ll need a powerhead and heater to mix the salt properly. There are a lot of salt choices out there but in my experience instant ocean is the best inexpensive one.