I think they did things equally wrong and should have been punished equally.
I don't think the "she was selling goods" argument quite flies. Is there a clause in your student handbook that says you're not supposed to give/receive blowjobs? I doubt it.... it's probably insinuated by some vaguer language about expected/tolerated behaviour. So the behavior on both parts is wrong and should be punished accordingly.
Your candy example: were you allowed to have candy in school (a lot of schools forbid eating outside of designated lunch times and areas). If not, then should your friend have been punished twice as much? He did something wrong, but the punishment has to 'fit the crime'.
Finally, I'm sure it just goes back to the double standard we women are accustomed to. If it would have been a boy offering "services" for $10 to the girls, do you really think he would have been given twice the punishment that the girls' were? I don't.
Did you say this was MIDDLE school?
The real issue is where are these kids parents and role models?