Grow a work ethic, for fuck's sake. You may be able to coast through school, but when you finally fulfil your ambition of getting into that particular university, you'll find your lack of motivation pretty much ruins everything for the first couple of years...
Also, just stop arguing with Mum. She's still wrong, but just leave her be. It'll be easier for you and everyone else in the house.
Save up some fucking money. I know it's great fun to spend every last penny as it comes, but I could really use some savings right now (so I could spend every penny on the stuff I want now instead
)
When you have to choose between fencing and choir, choose fencing. The choir master is a dick, you can still have singing lessons, and you'll end up quitting choir a year later anyway. Don't give up on the amazing opportunity and all the potential you have.
And of course, the obligatory "just go for it" advice about all sorts of crushes I had. Seriously, none of it will come to anything in the long run, but have a bit more fun for Christ's sake, you have more of a chance than you think you do, and having confidence in that works wonders.