Hi, wanted you to have a quick reply, though no direct experience of this.
The school reported because they care about you. If you are self harming then you need help from someone - GP, parents, CAMHS, someone.
Legally an adult is a moot point. You don't get full adult rights until you are 18 (eg at 16 you can't drive, or buy alcohol, or vote). You may feel you are all grown up, but tbh, you won't be until you are 25 or more.
If I had a friend who was self harming I would try to get them to seek help, or if appropriate, I would tell their partner, or sibling, or GP.
Please if you are self harming, seek help, and take care of yourself.
Best wishes.