OOO OOO How cool! You asked the right person! ;-)
When I was in middle school, I painted my room like a rainforest. Here are some pics. I don't know if you'd be allowed to paint, but it would be a fun thing to get everyone to contribute!
Even if you could just paint one wall... or if you have one of those large corkboards or a flat space you could buy something to cover the walls and then paint if you were so inclined.
Here's pics of my room if you want some ideas. Sorry, I made these 6 years ago and my skill has improved since then ;-D
Also for my room, I strung these butterflies I found (cheap @ Michael's) from the ceiling with fishing wire to make it look like they were floating around. Even better, you could have the kids each make a butterfly or other insect of their choice, or perhaps a bird.
You could get a few houseplants, they really add to the feel.
I'm sure you can come up with more! Just a few suggestions :-D