Didn't get the A-level subjects I wanted- what do I do?

I'm really struggling right now, I wasn't allowed to do maths Alevel which will basically ruin my chances of getting into astronomy in uni. The thing that really bothers me is I didn't do badly- I got an 8 (and 9s in the other ones I'm doing) but my school decide to tell me ON THE FIRST DAY OF SCHOOL, once I've completed the bridging work, bought the calculator, textbooks, everything... that they don't have the space to let me do maths and that it is oversubscribed. I go to a grammar school- how is MATHS oversubscribed? Surely they assume everyone is gonna do maths. And now I am forced to do chemistry along with geography and physics, and none of it was my fault? And I want to do astronomy in uni and this will ruin my chances of everything. I need to decide if I am changing schools by tomorrow, even though I am number 1 on the waiting list for maths, there's no promises someone will drop the subject if I stay at my school. So do I take chemistry (i got an 8) instead, and ruin my chances of uni, or move schools, lose my friends, and ruin my mental health, but be able to take maths.