That's terrible I'm sorry. I guess I would start by contacting your school district by phone or by official letter. Take responsibility for the mistake, and that you realize what a stupid mistake it was. Tell them you are deeply commited to your education and that you will comply with whatever punishment they see fit but please don't expel you from school. You plan on attending a college and not letting this terrible lapse in judgement ever happen again.

Stay proactive and on top of this....like today, tomorrow, Monday, etc. Show them your committed to completing your schooling.