I like the huge community fish idea
or the silver dollars.
Cheap, easy to maintain.
Catchy.
The only down-side, I can think of is the kids tapping the glass, scaring the schools into hiding, or encouraging them to hit the glass even more by seeing how well they school in reaction to the stimulus.
If you did, however, get a large monster fish, it might demand a certain awe and respect that would make the kids too scared to mess with it.
Bon chance.