I am in no way attempting to give the teachers a pass here, but this just occurred to me...
We know anecdotally that EB hurt herself while beating Zahra, by hitting her hand on the prosthesis. This indicates to me that much of the bruising Zahra endured may not have been apparent to an observer when she was dressed.
(I know I know, the black eyes would have been a huge giveaway! As an ex-teacher I have been down that path of reporting and it is no fun, but remains among my proudest memories from that career. If she showed up at school with black eyes, DSS should have been INUNDATED with calls--from everybody from Zahra's classroom teacher to the school secretary.)
Just thinking out loud...