Hey there summersea! :) I think you've got a fantastic idea there, wanting to set up a betta community tank for your kids to help spread the word and dispel one of those awful myths! And what a better place to do it then in a classroom? ;)
However, though you do have a lot of space to work with, personally I think your stocking plan might just be a little much. You've already got two schools of fish going, I would suggest instead of adding more schooling/shoaling fish(which Cories and Otos tend to be, they're both fish that enjoy company, Cories especially as mentioned previously)to instead just bump up the numbers you have, maybe even them both out at 8-10; 1-2 larger schools are generally better then many smaller schools/shoals.
Now, about the betta. While some can do very well in a large community tank and tolerate peaceful companions like the Tetras and Rasboras you have, it is in fact true that they are a solitary territorial species. In a tank that size with plenty of room for everyone and because the other fish are already there its slightly less likely you will have trouble, there is still the risk of whatever betta you choose just deciding to go after his tank-mates one day and, unfortunately, its impossible to predict if the betta will be aggressive or hostile towards tank mates or not......you'll only find out after you add him in.
So, it is a good idea to have a backup plan in place, just in case. Either at your home or at school, you should have a spare setup with the works(heater, decor, filter if 5+ gallons)on standby just in case things don't work out before you go pick out your new betta.
Also to help your betta feel more secure, you'll want to provide some tall plants that reach up close to the top or some floating plants of some kind(if you don't have any already)as in a larger space, being Labyrinth fish like the Gouramis, the betta is going to be primarily a top-level fish. The myth of them needing smaller spaces actually comes from a bettas need for proper planting and cover; they like a good ample amount of places to swim around and hide in and can get stressed or nervous with too much open space. Since it can be a little expensive to fill a larger tank with plants this usually leads to stressed bettas in larger tanks and most believing that they do better in smaller spaces instead, which is of course not true.
And as kind of an odd suggestion, though you don't absolutely need to do this, perhaps look for a choose a Plakat, or shorter-finned, male for your setup. Of course a longer-finned male can do just fine if thats what strikes your fancy, a shorter finned male will be able to get around a little easier in such a large space and you won't run the risk of ending up with a tail-biter.
But like I said, its just an odd suggestion; something I personally would do in a larger setup. You could certainly have a long-finned male without any problems in there if you'd like.
Last edited by DragonFish; 07-26-2013 at 01:27 PM.