Every fiber in my being is telling me it's wrong to share my advice on this topic, but I'm so conflicted because I remember being in high school (I'm in college now) and facing these same issues.

1) No more using your arms! Let those heal and wear bracelets. Bracelets are totally cute when you throw a few on together and no one thinks anything of them. If you need help covering scars that are still too obvious though, tattoo cover up works great. Just make sure you try it out at the store so you match your skin tone right.
2) While I don't want you hurting yourself at all if you have to-- inner thighs, hips, bra area, and shoulders are much easier to hide.
3) Buy thin long sleeve shirts
4) Don't hurt yourself more than once in the same area, it's much easier to make up an excuse if it's only one.

That's all I can think of for now, but I'll think on it.