You need to arrange a meeting with the school, and in the meantime, if possible, explore the (school / exam board) policy about 'challenging marks'.
Unfortunately it may not be as simple as the school changing the mark submitted, although if the school do admit they made a mistake it may be as simple as that!
I have heard that for a Centre (the school) to change a CA mark after it has been submitted / graded ALL
CA's for that subject would need to be sent to the exam board; i.e. the whole cohort's physics' CA's. The school are unlikely to be wanting to do this - and may NOT have to do this despite your protestations. There are very clear rules about this sort of thing and it is not always very easy to get something like this changed.
Good luck, and I sincerely hope it does turn out to be the school admitting an error and it's changed without fuss.