He's been asked many times to do workshops on writing but has never had the time in his schedule to do one of the type that I think you're asking about. He's done class visits but those were more an opportunity for students to do a Q&A, not an actual teaching class. I think his book On Writing was, in part, intended to be his Masterclass available to anyone.
Edit: He was going to do one for Maine students when John Baldacci was governor and a personal friend of Stephen's but we were never able to coordinate his schedule with when it would need to be done.