Changing schools mid A-levels

[Momsincharge]

What happens if you change schools from yr12 to yr13? Does it make A levels impossible? Are predicted grades impossible?

Why are you thinking of moving at that point?
From a parental point of view, it is something most expats try and avoid- moving countries mid GCSE or mid A level.
I'm sure it's not impossible, but it does make UCAS references and predictions harder. There are also issues with different examination boards (and hence different examinable content) along with different teaching order.

Might it be possible to start Y12 again in September if continuing at the current location is impossible,

Not a great idea as even if the same exam board is used the topics/texts chosen and the order of teaching may differ. You can restart year 12 but not all schools will admit out of year.

It happens. It can be dealt with. It's a lot easier if the student is motivated enough to catch up on a lot of the work themselves- it also depends on the subject. Sometimes there are a range of modules/pathways at A-level for a subject and one school may do one, and another another. So the students can just sit exams on what they studied in their previous school.
Another alternative is to go into school number 2 before the end of term and find out what work has been missed and make a start over summer.
In terms of predicted grades, we sometimes contacted the previous school to get them to do it.
Speak to the schools.

A solution is for your child to go down a year and start again in Year 12. This may be easier if they are a summer born as I think there is a funding change that happens when they are 19 in state schools (may be mistaken on that but worth checking).

Depends why. If bad teaching or bullying yes.
If because you are moving house and they are happy with the teaching : have you considered if they could board.with a friend? A friend did that with her eldest for year 11,12 and 13.