changing sixth form is it really helpful?

Hi,I ve been two weeks in since I changed sixth form from a normal faith school to a local grammar school.As I don't have much knowledge about how the system of school works in England and all UK ,could anyone advise me if I should stay at my old school(as all the teachers know me and willing to help me )or remain in the new grammar school(where the teaching style is better but then I'm am worried about my future a levels and if teachers will help me enough and if changing of school and coping with new teachers and classmates could worsen my sixth from experience ).also I wanted to apply at the best unis in engalnd and just wanted to know if you need to become head girl at your sixth form to get more chances to get accepted at Uni during the ucas application and what steps I could start taking now (year 12 )to become head girl in year 13 ,considering the fact that i am in a completely new environment and no one knows me .
thanks for the help