Ooo... the plan is underway! The exciting part! Just a thought ~ when I was playing around with the pond foam I found the it was easier to let it cure and then use black aquarium silicon to adhere stuff to it. The bond was much stronger, messy but stronger. On the tests bits that I played around with the ones that I used straight foam and sand on did not stand the test of time and it easily brushed off after it cured. I think from your earlier post you were going to use the foam & sand/gravel only? You might play around with it to see what works best for you. Other then that it looks like your off to a great start!
I can't wait to see what the art class comes up with - I bet it will look amazing. I love kids for their creativity and ability to think out of the box.
Keep the updates coming.