Hello,
Black milks have great temperaments... a perfect choice for a classroom full of young children. I think that as their price gets down to the range where beginners can afford them, they will become much more popular. Adult females are usually very expensive, if you can find them at all... I hope you got an educational discount! Anyway, to answer your question, that cage sounds a little small to me. Milks don't need to stretch out and climb as some species do, so height isn't too important. As far as width/length goes, I've read that it should be about 2/3 to 3/4 by 1/2 the length of the snake, so a 4' x 3' cage would be more suitable. Good luck!
Nate