I work at an addictions recovery house and we are currently calling our past participants "alumni". There's some debate whether this is the most accurate term as they aren't in an educational program nor do they graduate from the program. It's a life skills and wellness based program using both individual and group programming format. Once the participants re-integrate into the community, we maintain support and connection for a period of up to a year. How should we refer to those past participants?
While it's not a school, it's similar in that they get an education (in this case, how to deal with their addiction). And even though there's no formal graduation, completing the program and re-entering society is not unlike the way students complete high school or college and then enter the job market. College graduates often still maintain some link with their alma mater, it doesn't have to be a complete break.
So calling them "graduates" or "alumni" seems fine considering these analogies. I'm pretty sure I've heard "graduates" before.