Defamation- To defame someone/harm reputation.

You must have 3 elements, plus an additional element present in order to have a SLANDER case.

1) What was written/said about the person damages their reputation.
The coach telling an entire class of students that you are a user of a class III narcotic is obviously very damaging to your reputation.

2)Defendant asserted material as a FACT.
Since he did not say he THINKS your use steroids, or its ILLEDGED that you use steroids, he did assert it as a fact.

3)Things written/said must be the TRUTH.
As far as anyone else is concerned, you are not a steroid user. There is absolutely no proof or prior convictions for steroid use, therefore its not the truth.

*)Whatever was said, was said in front of a 3rd Person.
He said those things in front of your girlfriend, and to an entire class of students, so there is your 3rd party...

Obviously taking him to court would involve lawyer costs for you, plus trying to get those students to testify. If you're looking to pump the school district for some cash, Id say get a lawyer and go for it if you can get the kids to testify as to what the coach said. If you really dont care, just go meet with the principal like everyone above said.