I wonder if the meaning of 'classmate' can be extended to a person who goes to, say, English lessons as part of a short language course together with a group of other people. I'm asking because the word brings to mind the image of a school/college and, as far as I know, the word 'groupmate' (which I'd use in this context in my native language) doesn't actually exist in English.
That use would make perfect sense to me, as long as you specified which classes/program you had together.
If that seems too strange, you could say "a fellow student at..."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.