Hi, I probably started to self harm almost the exact stage you are at right now. I was also going into sixth form. I can't tell you what to do cause it's about what you feel comfortable with. I myself did hide them, because I was embarrassed and ashamed, but by that point I hadn't actually stopped. Once I stopped in year 13 after my parents found out- I got better at just accepting that they aren't going to go away completely and that actually there are a lot more people that deal with it than you'd think. One day I was exposed (admittedly I didn't even realise my sleeves were up) and the girl sitting next to me in class who I'd never spoken to before was actually dealing with the same thing. We became friends after that and were able to talk to eachother about our sadness. If she hadn't have seen, I might not have had a friend to talk to.
So I can see both sides. Don't get into the habit of hiding them because you'll always feel like you have to, and it can make your self-esteem very low, like mine was/is. I've been clean 3 years almost but I still bear the scars and still hide them. I don't want you to be like that so do whatever you feel comfortable in. The summer is not a fun time to be wearing jackets and long sleeves I admit, so keep that in mind!
I hope you have stopped or are seeking help, and hope you begin to feel happier. Good luck in 6th form- the beginning of the rest of your life! x