I haven't read through all the responses, so I might be repeating what someone else has already said, but I've got just about the cheapest light/lid set up you can imagine...and it's worked with both higher and low light plants.
Just get some plexiglass or actual window glass (just make sure the edges are sanded or otherwise finished and not sharp) at the hardware store, most places will cut it for you to size. Leave about two inches on either end of the tank for filters, heater cords, air exchange, and to have room for the light fixtures.
This should run you about ten or fifteen bucks for a single tank.
Then, get two clip on, flexible, desk lamps. I use one for each end of my 20g long tanks. Get **DAYLIGHT** (this is vital, the regular lights work, but the daylight really works much better) 75 watt equiv CFL (the compact, curly fluorescent lights like you'd use in your house), and put those into the lamps. Bend the lamps so that they're pointing directly down into your tank, and you're good to go.
It's great because it lets in plenty of light (including any overhead light in the room, which is a plus as far as I'm concerned!), and you get a lovely unobstructed view of your fishies.
Plus, I think it just looks really clean and nice...and I am just not a fan of those big old fashioned black hoods. Totally personal preference though!!
Here are some photos:
This one was a complete overhaul...I had some plants in there that died, still working on getting it to grow in/fill out to where it should be. Also, please excuse my messy bathroom...it's also in the process of an overhaul!
Camera glare makes everything look cloudy, but you can see the lamps and how they clip on.
Due to camera glare off of water, you can't tell as well as I'd like...but it's awesome being able to see your fish from all sides, *including* the top!
One note about plexi, is that it DOES warp when exposed to the heat of the lamps. So, I turn it over once a day to help keep it in shape. But, even if you don't, nothing bad happens, it won't melt or anything.
I used scrap pieces for this tank, so the fit is not ideal. I am going to "upgrade" to glass tops, I just haven't gotten around to it yet. Since you mentioned you were in a classroom, plexi might be a safer choice for you though, since you don't have to worry about breakage.
Good view of lamps. On this piece I used a section of a build-it-yourself shelving kit (the black "mesh" to see if it would help the warping...it doesn't, but that's what that is). Not at all necessary, I just wanted to see if it would help.
This tank is the new sorority (upgraded from a 30g to a 40g breeder), so the acrylic doesn't fit all that well, and the plants I've ordered for it have not yet come in. So, for now, it's only partially planted and partially silk...and the arrangement still needs some work. But that's all beside the point, this one shows that you can do more than one lamp on each side if you need more light or have a wider tank.
Last one! Just the top view of the 40g with the lamps and more of the black shelving unit pieces because the acrylic isn't wide enough for the breeder width. But, again, gives an idea for lamps and placement.
I got these lights at walmart in the student/dorm section, and I believe I paid about 7$ each for them? They're cheap, which is great when you're buying twelve of them, lol!
So the entire setup for each tank was about twenty bucks, total? You'd be hard pressed to get a better deal than that, lol!