The tank will cover a fire extinguisher cubby? As in you can't access it unless you go through the tank stand or move the tank? You really want to rethink that because that is all sorts of illegal (more so because it's a school).
That said, try to minimize any sort of bubbling so that you don't get saltwater spritz, humidity might become an issue depending upon the tank size but if you can control humidity and spritz you won't have to worry about corrosion as much. Also as long as the metal has some sort of coating on it (paint, sealer, etc) it'll hold off corrosion pretty well, the places you are kind of screwed with are any sort of moving parts like hinges.