Why are they killing children in Nigeria?

We are continually being told that Islam is a peaceful religion, that Muslims are non violent, amicable people. Why then, are Muslims killing Muslim children in Nigeria? Apparently just because they are going to school! Are these killers worried that educated children will see them for what they are and won't want to join them in their cause, but just might want to go on to become teachers, doctors, nurses or some other worthwhile occupation? Yes, I can see why militant rebels would not want anyone to get an education.