Simply put, it's a waste of time you being in Biology lessons if you don't like the subject. You won't be motivated to put the work in, to revise, and ultimately to succeed.
Have you explained your predicament to the person in charge of changing subjects? They may still allow you to switch if you missed the deadline because of your mother's refusal, and delays stemming from that. Your sixth form should allow you to change regardless of what your parent says anyway, after all you are in further education now.