DSS12 has hit a similar stage of behaviour - although he's not upset about going to school.
Social media has lower age limits for a reason – I feel quite strongly that children don't have the boundaries and critical faculties to navigate social media and it has a huge effect on self esteem.
A school move in and of itself is stressful and he'll be trying to fit in and find his friends, as you've said. Give him some time and work on building up his self-esteem in ways that don't rely on external factors like looks and trappings.
We're in the midst of this right now with DSS and so we're reducing social media and we've started volunteering and taking part in activities that he can take real pride in.
We've also been practicing and role playing different social situations in the car and when we see interesting dynamics on TV: "what could X have said to that?" "Do you think that was a smart way to get out of that argument?"
I'm also making a point of saying I absolutely believe everything DSS says, to the point where he 'gets' the consequences of lies – he told me something about one of his teachers that he thought would make me laugh yesterday, was clearly a lie, but would have lost the teacher their job if it was true. I could see he was lying, but I told him I was taking it very seriously and would be setting up an appointment to speak to the school and possibly the police, and when we got home I'd need him to sit down with me and write down exactly what he saw... didn't take long to backpedal from there.
We'll see how it goes, but fingers crossed for you and him – I think part of this really is just an age thing, and this particular age in this particular generation, with social media etc., has a real emotional onslaught.
Hope things go well at school today - will be thinking of you all!