M.M.
I promise you this was me about 6 months ago. And still me a few days out of the month.
I had the same problem when my 7y/o (now 8) was acting up in her 1st grade class. I didn't understand why or how this behavior came about, because she was perfect in her previous year (grade K). Perfect grades and conduct. But oh, come 1st grade and the teachers were calling every other day, notes daily, and referrals at least once a week.
Finally I took her to the pediatrician, who informed me she had ADHD. And I cried like a baby because I just knew he was trying to tell me my baby was mentally ill or something. But putting her on Concerta (med for ADHD) I haven't had anymore problems (except when she misses a dose). I am not trying to tell you that your child needs medication, I never wanted my kids on nothing but a Flinstone. But for some reason THIS is the only thing that works for us.