Probably your best bet is to get a hold of your school district or state's Department of Education and do a group order for those.
As far as I know, there are no cases of that type for a screen that size available.
Your school will most likely not be the only one to order these tablets and a group order would be more practical, plus, the more you buy, the more you save.
The cases are a wise idea and would protect the investment that your school made. Other districts would benefit from that as well.