when abhay hit kshitij he was sitting but when kksum comes back he is on the floor head down.He must have been unconscious and Mahi may have done something in the meantime .there is something fishy bcoz she is taking too much interest in implicating kkusum and abhinav.Also abhay is the hero and nothing can happen to him(like jail etc)
abhinav could not have done it bcoz he was with kkusum all the time at school with simi