Clarissa was trying to pass the time by drawing something insane. Then she wants to draw on the other kid's cast because kids like to do things other kids are doing. They don't want her to, and tell her to break her own arm so she can get a cast to draw on. Being an imaginative child, she thinks her "force field" she mentioned earlier is real, and slams her arm against the copier trying to prove she can't hurt herself, but hurts her arm anyway and upsets the office lady.
Cue scene transition.
It's frankly the most innocent moment in all of the Clarissa comics. It's just Clarissa wanting to fit in like any child would, and throwing a minor fit when she's denied that. Nothing sinister or anything, just kid stuff.