Doing a-level biology privately.

I wasn't able to get the grades I needed to do a-level biology and chemistry and I can't apply to other schools now since enrolment is over. I am planning to do chemistry, sociology and law, and then do biology separately. If my school allows me to do 2 a-levels then I will be dropping sociology. I just wanted to ask if it is possible for me to do most of my a-levels at a sixth form and then one a-level privately?

Do you need biology for the course you want to study at uni? If you do, you can always do the foundation year version of that degree. The extra year is to build your gcse knowledge to the A Level standard. It is unlikely that the school will allow you to do 2 A Levels, but email them your concerns.
Doing it privately, will give you less support, and cost an arm and a leg. You will also have to register as a private candidate for that exam, and do it in an assessment centre that is not your school