She needs the wifi access to do homework just as much as she needs it to do her social networking. Short of standing behind her looking at the screen permanently, there is no easy way of controlling when she is using the wifi access to work and when she isn't!
My 15yo has an ipad mini that she got through school. Hove Park now dish out homework and study notes on the ipad and expect submissions electronically.
It's now almost impossible to tell if she's glued to it for good reasons or because she's permanently on facebook/facetime!