How do sixth forms accept students when theres hundreds of applicants?

My first choice sixth form is a grammar school, and I am an external candidate. The school provides 250 spaces in Year 12 and they did say 80 external students were accepted last year in Year 12. My question is, do sixth forms accept only the top applicants for that external place?
The school mentioned they had received over 500 applications. The basic criteria is: an APS of 6 over top 8 subjects and grade 7 on chosen subjects. I would imagine most applicants would pass this basic criteria — so how would sixth forms decide who would be accepted? The students with the highest grades on results day?

thanks guys! turns out the school i’m going to prioritises students in difficult situations, and then those with the top grades. i’ll try my best!!