Please don’t ‘tighten your parenting’ - be on his side. If his whole demeanour changes when he goes through the school gates don’t make him go through the school gates!
if he’s a teen and is still undiagnosed he will have lived all these years being treated (held to to standards) of someone whose brain isn’t neurologically disabled. It is a disability. And it is disabling. It is literally a government acknowledged disability.
so imagine being told your whole life that you’re doing things wrong but you don’t know why. Then that you SHOULD behave like this and you SHOULD talk like that but you’re just being yourself - what happens is you eventually decide to stop being yourself. but now you’re angry. And sad. And misunderstood. On top of that your dopamine doesn’t work properly (for want of a more elaborate medical description) so you get a kick from all the things you shouldn’t do - arguing for instance, causing reactions. plus frankly you’re angry and want to piss your teachers off.
I wrote here the other day that basically it’s like that part of your brain is stuck as a child. You don’t get any kind of satisfaction from things that other people do. You’re happy only when you’re in a fantasy world. You can’t prioritise - messy room for months then randomly decide to tidy when you’re meant to on your way to an appointment? Little habit forming ability. All these things youd see in a 6 year old child.
also, and I can’t speak for boys because I know it can be different but go on TikTok and find this is what adhd sounds like videos. Because it’s actually pretty accurate. For me it’s louder than they show on these videos but still gives a good idea. It’s very loud inside your head but you don’t think it is abnormal because it’s normal for you - medication showed me what it’s like not to have 15 different conversations and 3 songs going on inside your head all day long. How can you expect someone to concentrate when their head is full of noise?
adhd kids get told off by teachers and parents a lot more than non adhd. Their friends also notice that they are different and make comments. If your son is taking on this persona, I’d highly recommend not enforcing that this is who he is by punishing and agreeing with teachers that he is badly behaved.
of course he needs to not do harmful things, but then maybe he needs to be out of that environment and have a diagnosis pursued and have his attention put elsewhere.
it’s likely going to be super rough until he’s about 25 but you can definitely try to intervene and prevent as much as possible by being his advocate.
sorry that’s so long (and I know it’s really hard as a parent not to just want to shake them believe me I do) but from the child’s point of view the more alone you feel, the more people tell you you’re “wrong”, the more it just enforced what you’re already thinking - that you’re weird and wrong.