The principal's "I don't care who started it" can be a poorly-phrased "both of you started it." In every case, each kid will put full blame on the other--how often do you expect to hear "he started most of it, but I'm responsible for some of the trouble as well?" Often, both kids will even believe what they're saying. But in almost every case (perhaps excluding the playground bully), both contributed to escalating the conflict. Anyone who has shepherded groups of children can confirm this, and it holds just as true for adults, tribes, and nations.