Sorry Annie to hear about your son - not sure how old your son is, but if you mention hormones kicking in I am guessing around 13ish.
When my son was just going into year 9 at secondary school - he refused point blank to go back after the 6 weeks holidays. To cut a long story short, I never did get to the bottom of it. I know it was not physical bullying, because my son can handle himself, and it would have been sorted - but all sorts was going through my mind, from mental bullying to sexual abuse by a teacher.
In the end, he was off school for 7 weeks, I was threatened with court action, but stuck to my guns and removed him from the school. We went on appeal to get him into another school, which we won. My personal opinion was he was having problems with mentally bullying from someone - as you say with the hormone thingy, they are finding their feet and can be quite sensitive to cruel comments.
Bypass the teachers and take things into your own hands Annie - it will be the only way to get a result.