you really wouldn't need much space for the plants, even something like routing the flow through a section of gutter filled with hydroponic substrate would probably work pretty well.
Though, if you go that route, it would probably work better if you split it into 2-3 sections with individual lighting on timers, and keep each section lit for 8-12 hours or so. -that way there is always a section in 'active' mode.
I've never drilled a tank personally, but it sounds like it is a bit much trouble to do if other options are available. I'd go for just sticking a small pump in a corner, having it push water up into a length of gutter (or three), and the over flow from that just goes back into the tank. pretty easy set-up, and doesn't require much more then something to cut the vinyl tubing, and whatever glue/sealant you use to set up the sections of gutter.
Aside from that, kudoz/props/whatever for trying to bring a relatable aspect to science education. I sincerely think something like a classroom fishtank, or a walk through the woods where all kinds of rocks and logs are flipped over will do more to stimulate kid's curiosity(and curiosity is the root of all science, and even knowledge) then much of what they will otherwise be exposed too.
And, hell, get the kids involved in the design once they understand what is needed. they will likely spit out all sorts of bad ideas, but there may be a few in there that we haven't considered yet, and most bad ideas have value as teaching opportunities.