If a GCSE option is oversubscribed, what determines is a student gets in?

Okay so to provide some context I've just picked my GCSE options and literally next week are going to be my end of year tests which decide what set we get in for year 10. I would be begun revising earlier but they only gave us about 2 weeks notice for these tests. We're going to get one test for each subject, but the problem is they haven't confirmed our GCSE option yet. My school keeps threatening us by saying that we need to revise equally for all of the tests in case one of our options is oversubscribed. My school also keeps saying that apparently our grades we get now in year 9 are going to be important when we apply to college. The thing is due to the amount of notice they've given us I only have the time to revise for the 4 subjects I've picked and my core subjects or I risk failing one of the subjects I've picked for my GCSEs. How do they decide who gets into a GCSE option or not if its oversubscribed? Will I not get into a good college in years 11 if I completely fail my year 9 RS test but then do really well on history? What do I do?

I think your school are telling you stuff to make you revise! Marks in Year 9 are not used for college entry. College will want GCSE grade predictions.