TBH any teacher confronted by 'Miss! Child x is being naughty' should shut that right down. DS has had some great teachers and some not so great teachers, but even with the not so great ones, that's the kind of scenario they'll close down with a simple 'thank you child y, please get on with your work.' A teacher really shouldn't be looking to explain any behaviour to another child:
Miss, child X is being naughty!
Yes child A, that's because his parents were drug users who neglected him and now he's being fostered, which means we all have to be a little understanding.
Miss, Child Y is being naughty!
Yes child A, that's because his dad just died, isn't that terrible? We'll all just have to cut him a little bit of slack.
Miss, Child Z is being naughty!
Yes Child A, that's because he's a little shit with the most annoying mother I've ever come across in 25 years' of teaching. Unfortunately she's the HT's best pal, so we'll have to cut them a little bit of slack.
They shouldn't engage or explain. Tbh there are so many different needs in a school these days that if they engaged or explained in every situation they'd never get anything done!
I honestly think you're overthinking this part of it, but I see that I'd missed how new the diagnosis is - you don't have to decide anything just now, you can just sit with it for a bit and absorb it, before telling anyone - DS included 