First off the teacher has no choice but to report. The teacher must report if s/he suspects abuse or, if told of abuse, even if the teacher knows that the story is bogus. Even reporting to the principal ( at least in Wisconsin) does not remove legal obligations.
Second, most coloration of brusing does not take place immediately, a yellow coloration is quite an old bruise usualy at least a week. Also, a kid that is in a lot of fights at school can quite possibly be someone who has been taught at home that violence is a way of solving problems (ie abuse), so a kid that had just been in a fight would not be excluded from having also been abused.
Thirdly children who are disabled are more likely to be abused. ADHD kids are substantialy more frustrating than normal kids, and everyone has all sorts of absolutely impracticle advice involving 24/7 suppervision and the patience of a saint.
That all said, slapping the kid in frustration isn’t the answer, neither is a 12 minute time out for a 12 year old ( I can’t really believe they said that) There is help out there. It is hard to find and can be contradictory (these kids are all wired a little differently and not all that wiring is the same so the same answer doesn’t work on every kid), but it is out there. Maybe it is time to see what can help, I suspect there are several people on the board who can point your friend in the right direction.
I am fortunate. My ADHD kid is pretty well helped (most of the time) with drugs. With the Stratera we are even winning the homework battles about 60% of the time, that doesn’t mean there aren’t still times I get the urge to slap her silly.