If you suspect that the teacher is using illegal drugs, you need to let the administration know immediately.  Since I don't know if you are a parent or a student, I would suggest that if you are a student to discuss this with matter with your parents.  It doesn't matter who you are, if you suspect a teacher is coming to school under the influence of an illegal drug, then the administration should be aware of the situation.  

The first thing you could do is write down all your concerns, sometimes putting events in a timeline can be helpful.  Stick to the facts even if it is something you observed.  Ask to have a conference whether in person or over the phone with the principal or assistant principal.

Most school systems have employee assistance programs where they can get the necessary help.

Hope that helps.

Barb K
www.LessonPop.com