I've read the comments here with interest and thought I would contribute my thoughts. I teach Biology 11 and Junior science and am currently running a 29 gallon biocube, and 2 - 14's in the classroom. I have had them running since last year and the kids find them fascinating. I actually gave quite alot of thought to what I wanted to do before I started stocking the tanks and this is what I wanted to accomplish.
1. I've had freshwater tanks in the classroom in the past and have found erasers, pencils etc in them so a) the tanks are located at the front of the class and b) the tops are covered. This also reduces evaporation.
2. Smaller multiple tanks are easier to breakdown and transport as well as for setting up different themes. The downside is I need alot of outlets for electrical and have had a 14 gallon get wiped out when the breaker tripped over a weekend and I lost filtration and heat.
The 29 is my display tank with heavy emphasis on the inverts to correspond to the Biology 11 curriculum. I've got a pair of Bangaiis (mouth brooding would be fascintating for the students if they get to that point), a mandarin (the body shape with large pectoral fins for hovering and small mouth shows adaptations), an anemone crab to show symbiosis as well as filter feeding, a tiger tail cucumber that deposits sand pellets as it feeds, and a variety of softies like pulsing xenias and LPS that I can feed to show that they are animals. The softies are also great for talking about things like palytoxin and how tropical cnidarians pack more punch than our local stuff. I take my student to the intertidal invertebrate program at the Vancouver Aquarium for the touch tank experience so they don't miss out on the touching inverts part.
The theme for one of the 14's is symbiosis between a pistol shrimp and goby (plus the action of the pistol "cracking" is a lesson in physics). I started out with what I thought was a pair of high fin black ray gobies and a Randall's pistol but the gobies had a squabble and one ended up in compartment 3 in the back and I haven't been able to get it out. I lost sight of the Randall's pistol recently so now have moved the lone goby to the 29 since the biggest complaint of students is the lack of "fish" action. Fish are more immediately noticeable than inverts, especially colourful fish and sometimes that's the hook before the students become more fascinated with the inverts. On that note, the cardinals are great since they don't hide in the rockwork as much as some other fish. Students always want "nemos" but I've had a few too many clowns that didn't get along with each other so I am now staying away from them unless I go with just a single individual. In the other 14, I had purchased a tiger pistol and will be getting a partner goby for it soon.
Haven't decided what to do with the vacant 14 at this point and may sell it to finance other acquisitions (the school does not chip in any funds). Some options may be a small angler. I've had a couple before and the fin adaptations and spiracles instead of gill flaps amaze students as well as, of course, the carnivorous predator action.
I also have a nice acrylic 29 gallon sitting empty that I 'm saving for a peacock mantis when one becomes available. I've had one before and the destructive power of its smashing apendages is awe inspiring! (Did I mention, I'm running out of outlets.
The students still need some for microscope work!
Anyways, that's my 2 cents.
If I get a chance, I'll post some pics.