I'm pleased to report the meeting with DS1's high school principal, went well.
The principal (TP) was genuinely shocked that T hadn't been disciplined for the comments he made. She said to me, that even if T was 'only joking', the comments are still inappropriate for a classroom environment.
TP told me that it was right that DS1 was spoken to, for calling T an idiot, and I assured her I wasn't complaining about that. I said I'd spoken to DS1, and told him he was right to speak up, but should have not have resorted to an insult. I told TP that DS1 has acknowledged this and even apologized to T yesterday, in art class.
I explained to TP that my gripes are that (a) T wasn't disciplined at all, for using far more offensive language than DS1 did, and (b) how casually Mr Health Teacher dismissed T's comments as not being serious, when I phoned him to speak to his letter to us about DS1. I added my main concern was the attitude of the Mr Health Teacher, and TP acknowledged that he had not met the standards required of him, as a certified health teacher.
Anyway, I won't bore you with everything that was said, but TP said she would follow up and get back to me by the end of the week.
I came away feeling positive.