Have you ever heard of something called rejection sensitive dysphoria?
It can be linked to autism but isn't a profile of autism in itself, so many people can experience this.
It is an extreme emotional response to perceived criticism, rejection, or failure.
It goes hand in hand with pathological demand avoidance which is again, like RSD, linked to autism, though can be recognised on it's own. There isn't a clinical diagnosis for this in the UK though, and so it would just been reviewed as part of an autism assessment and observed.
Pathological demand avoidance is an extreme anxiety driven avoidance of everyday demands and expectations. PDA responses aren't behavioural. They're neurological, and are not a choice.
The anxiety doesn't come from the task itself but by the expectation to complete the task, and it triggers a fight, flight, freeze or fawn response.
There are external demands: put your shoes on, get in the bath, pick your toys up.
Then there are internal demands: you're hungry get some food, you're thirsty get a drink, you haven't been to the bathroom for hours go pee, you are really bored of this tv show change the channel etc.
When these demands are placed on people with PDA, it feels like a direct attack on their autonomy.
People with PDA might seem socially capable, but struggle with executive functioning tasks, following instructions and accepting help.
Then on top of that if you add in masking, which happens so often in schools or workplaces. You spend all your energy pretending to be unbothered, more capable, holding yourself to high standards, that you've got no bandwidth to continue at home.
Someone mentioned to me a while ago something called spoon theory. Basically everyone starts the day with so many spoons, which is your currency for getting things done. For NT, healthy people, each task costs less spoons, but for ND or chronically ill people, you have to pay an additional tax of spoons per action. This means if you're ND, you will run out of spoons faster than your peers. Usually you use these up before the day has begun or while you're at school and when you are finally home, you're in spoon debt, and then people still ask you to do more and even the smallest of tasks can feel like a struggle and a panic because you're eating into tomorrow's spoon allowance.
For people like this the best way around this is a lower demand lifestyle. Less toys, less clutter, more routine, eating the frog and getting the hardest tasks done earlier in the day, sensory aids, really paring down on what is around so you have necessities easily accessible and luxuries tidied away, less trips out of the house, reduced additional activities, priority lunch queuing, amended timetables etc.
I'm not a psychiatrist, I don't want to armchair diagnose anyone but I am just commenting on what I've observed in your post.
I do think these transitional changes, monotropic views, extreme reactions to perceived change, struggles with executive functioning etc could have grounds to be investigated. I say this as someone who has undergone the autism pathway for myself, my son, and now a late diagnosed relative.
There isn't enough information in your post to highlight the triad of impairments, nor should there be because this is mumsnet, but I do think personally that there could be a neurological reason behind these responses as opposed to behavioural responses.
Also don't beat yourself up about being a teacher and feeling like you should know. Nobody knows everything and our understanding of neurodivergence is ever evolving so it's hard to know anything for certain at this stage. My own mother works with neurodivergence and I was only diagnosed last year! Everybody is so different in their presentations that it's a constant process of learning and relearning.
Wish you all the best.