College in S4

Hi I am in s3 rn and in a month am supposed to start picking my subjects. I do physics, biology, computing, geography, drama and pe masterclass. I want to drop drama and geography but I need another subject so am stuck between sport leaders or college and I really want to do college but everyone is telling me not to do it cuz am missing alot of classes. I just don't understand how the timetable works cuz college is one of my subjects so how am I missing classes when there's supposed to be college at that time. Can someone that choose college in S4 tell me how it was?