I got a B's at A-level (bio,chem,phys) in 1991. Did nothing scientific until 2005 when I trained to be a (general) science teacher. I only opted for chemistry when I got a job. Day one the following September I was teaching Y13 chemistry to a class of kids wanting an A*, using nothing but my memory of chemistry and what I had re-taught myself from books.
Essentially, what I'm saying is that with a bit of motivation you will be able to do it. Hopefully, you'll be rather more mature than most of your classmates and will have chosen to be there rather than just falling onto the course 'cos their 16 year-old heads don't know what to do otherwise. You will, therefore, be a bit more disciplined about extra-studies.