That does sound disturbingly familiar, ---- though in my case it wasn't just one person who began it, ---- and most of the serious Sa was perpetrated by girls.
My parents I think only worked out the Sa from my behaviour, flinching at the S word, difficulties with physical contact and a distinct fear of being seen in any state of undress.
I also got generally good marks in school, I actually looked forward to all exams tests and mocks, sinse I got extra time for being visually impared, I took them elsewhere and thus didn't spend any time with other pupils, but bad marks as far as related to others went, ---- not surprising as basically people were devided into two groups, those who did things to me and those who didn't.
I'm really glad your son moved schools and was spared all of that. I'm 26 and only now dealing with the consequences myself, which is why I'm here on this site.