Don't be afraid of using up lunchtimes or free periods to catch up with work, because you're worried you may be missing out on fun times with your friends. That was one of my main worries (since GCSEs), but recently I've started to use most of these periods of my time, particularly for coursework, and I'm not afraid of missing out nowadays! You can still have plenty of time for your friends and your own stuff (even if it doesn't seem like it), and you'll thank yourself later when you can do something without a rush of guilt.
I suppose this is something that we should probably be doing anyway, but I know how much of a problem this can be for some people. Good luck!