makeup? or cant you just put a cardigan/jumper/coat on?

I sympathise with you. I had a few unpleasant inncidents when I was in High School because of this. Teachers & school officials reported my scarrs & cuts & even threatened my mother that I would be taken away from her. Be careful that the same doesn't happen with you.

sadly, i've gotten to the point where i don't care. the only person i hide it from is my parents.
my best recommendation would be arm bands? i take random fabrics and wrap them on my wrists and just say i felt like wearing them.
also, keep a jacket handy. even if you just drape it over your arm? that's what helped me alot... as for your shoulder, makeup should do. are they really bad?? cause i did it on my shoulder and didn't even really have to cover them....
anyhow, a jacket and arm bands is something i would recommend.. works for me.

you're not stupid, triggers happen when they happen, i've had to leave class so i could cut. as for hiding the cuts
-long sleeves
-bracelets
-jackets
-armbands
-arm warmers
-when fading-makeup
you can bandage your forearm and blame it on an animal or fall or other accident. -if you're a klutz like i am its a bit easier to believe. good luck