What Ronnie said - Try and discuss with whoever is the authority over letting people take the course. I'd also suggest speaking to your previous biology teachers and trying to sway their opinion of you as a B is not considered poor by anyone. Also let them know that by not allowing you to do biology, they're lowering his chances to apply for medicine in the following year. If they;re pricks and still won't let you, look into switching to a more lenient Sixth Form/College (my one only required a B to do either science/maths)
You can do it externally but leave that as a final resort.