An added advantage if the standard of teaching is really good so you're likely to get good grades but that's about it. Getting good grades vs getting into Oxbridge are not the same thing as everyone who applies has good grades. There's plenty of very high achievers at non-selective state schools and Cambridge will not favour you because you attended a grammar school. It's not about what school you went to, it's about how good you are and how much you know about and love your subject. Going to a grammar school means nothing, they give offers based on meritocracy not based on what college you went to. My state college is non selective, and we get about 15 Oxbridge offers a year. It's about the individual, not the school. If you're smart enough and love your subject enough, you'll get in regardless of your sixth form. Good luck!