Well lets look at the evidence. We have eye witness testimoney from the teacher that Millie hit Jeremy. Millie hasn't denied hitting him either. We have mud on Millie, but she also has a history of activly playing in mud, and we have her best friend in the world who is not a reliable witness because he is bias. Given that the teacher has little to no time to gather more evidence or testimoney I feel that there is little other conclusion she could draw from the situation.
Other evidence: Jeremy's history as a known and violent bully. I believe Ms. Mudd sued for that before. Millie's history as a peaceful person. Ozy being another witness - just becuase he's her friend doesn't make him unreliable as a witness, especially given the fact that Jeremy has been bullying him for years and that fact is also on record at the school.
Given all that, in addition to the size difference between them, and at the very least, you have a case of obvious self-defence. I'm certain that Ms. Sorkowitz has at least a basic understanding of the natures of her students, and that alone, in such a situation, would be enough for her to at least consider that there may be more going on that what seems obvious.
Notice - she never wondered *why* Millie hit Jeremy. She's only reacting to the last of a sequence of events - as I said earler, that's not something you can afford to do anywhere, especially in a school. Unless she perhaps is already on medication for that? In which case, she needs her dosage looked at.
