Will going to a below average sixth form ruin my A level grades?

I don't know where I want to go for sixth form and all my friends and many people in my current year are thinking about going to Sixth Form 'A' which is also my first choice. However the grades and results are below average : the average grade is a C when most schools is like around a B- , also the percentage of students achieving above AAB is only 9.8% when most schools is like around 17.5% . The school looks nice and I'd like to go there but the grades are putting me off. The Sixth form is massive - it has around 1300 students in years 12 & 13 (there's no secondary school) and the headmaster said they don't kick anyone out of courses even if theyre failing - so could this be why the grades look so bad??? I'm trying to convince myself it's okay D:.

TO SUMMARISE - can I get high grades of atleast an AAB at level even if the teaching is less than ideal? thank you