I would say sixth form 1! I went from a state to private (which isn't exactly a grammar but they do have their fair share of similarities), and left my friends behind. The school environment is completely different and generally the teachers care about grades and statistics for their department more than anything. Also, I found that at private school, when you struggle you generally stand alone - teachers really like to pamper the Oxbridge hopefuls and leave everyone else to trail behind. Sometimes it's easier to stick with what you know.