Don't worry about everyone else, worry about yourself. Do whatever it takes to make sure you are the best you can be. Cambridge sometimes takes into consideration circumstance, but only really at GCSE as you apply without having gotten the actual A level grades. If you can prove how much you love your subject, and have the predicted grades to back it up, you're likely to get an offer from Cambridge - getting good grades isn't always enough - it's about your enthusiasm and knowledge about what you want to study. You can apply wherever you want, your college can't stop you. If you get an interview, they like you, you get an offer and you get A*AA (or whatever your requirements are) you'll get in. It's about the individual not the sixth form, if you're doing well now, you'll likely be doing well in 6 months time. You may have less support in terms of writing an amazing personal statement etc, but I'm sure you'll be fine.