Contact the school administrator.  You can either call or drop off an anonymous note.  However, I encourage you to not be afraid to identify yourself.  (That would be a great lesson for your child.)  If you do, just ask the administration to keep your name out of it.  If you don't trust the administration, then contact the police to investigate or school board.

When you notice the authorities, stick to the facts.  Date, time, and exactly what you saw.  Don't add your opinion.  If you think you saw something, then say, "It looked like..."  If you know for sure, then say, "It was..."  

Most school districts have services available to help employees who have substance abuses.  You want to get this teacher help.

I would also take the time to talk with your child, since he/she witnesses the situation.  I would encourage you to ask you child to not talk about it at school or with his/her friends. You do not want to start rumors or have people panic if there isn't a formal investigation.  

Good luck.

Barb K
www.LessonPop.com