Number two is the obvious one.
One thing I did with my kid is tell her...... tell the teacher every time somebody is doing something wrong. Do it right away. Most teachers, at that age group, knowing the kids, can tell whether it is the truth, and can get the truth out of the kid(s) who might be bullying. Usually it is a tears and lesson issue with times outs, card turned, parents called, and all of that. It might take a few times and even a trip to the Principal (trauma and drama at that age) but they can be reigned in.
These are very young children..... they need to be held accountable but are not really at fault... they need to be taught.
If the issue isn't resolved then you need to get your kid out of the school.
As far as the brain concussion issue...... I might be the wrong guy to ask.... my kid had a TBI that has never been diagnosed, left her partially paralyzed and basically needing 24/7 care for months, and while physically 100%, still can't read at more than 30% of her previous level (up from 3% last year so there is hope), and to this day has not been diagnosed (which means no ESS services, extra time to take the SAT, and so on). Neurology is in it's infancy yet neurologists think they know everything.