Oh this is incredibly tiresome, isn't it? First having to manage your son's behaviour, other people's behaviour around your son, your son's reactions to other people's behaviour, and other people's emotions around your son's behaviour. It's exhausting.
What helps me is to think of school as a bargain between the parent and the teachers. I send my 14 year old with SEN into school fed, clean, uniform on, as rested as his conditions allow, with his pen, his pe kit and his planner.
That is my end of the bargain.
If he hurts someone, I expect them to let me know. If the keep him in after school, I need to know. But other than that, they need to manage his behaviour without bothering me with detailed minutae such as "Ds1 didn't tie his tie properly after PE" "Ds1 wouldn't stop repeated the word 'cheese' during history and has been given a ten minute detention" "Ds1 had to be reminded to stop chewing his pe bag"
Because it really is minor, low grade misbehaviour that his TA is funded to deal with.
With your son in a special school, they are funded to deal with low grade misbehaviour. If he has ASD and is diagnosed, I would suggest asking Autism Outreach for help, both for yourself and the school, in terms of their expectations of the control you can exert over a 15 year old with clinically significant behavioural problems.
And do remember that many of the posters on MN don't even have teenagers, let alone teenagers needing specialist provision. Take all the pearl clutching with a pinch of salt and consider letting your phone go to answerphone in the day