Sex is the one thing that tops/seals you and your girlfriends's love, if you've done everything and ready for more to connect each other then go for it. At the end of the day, people might say keep to your year group, but you may be one of the youngest in say year 11 and the girl might be the oldest for year 10. I've been out with a girl two years below me, but the relationship wasn't anything sexual. And for anyone having doubts and thoughts on it, if shes mature and no slag, go for it.