I think they will not all s1 cast will comeback on Study Group 2 since few actor schedule are overlaps with the…
the actor who plays Lee jun recently posted some selfies on his Instagram account with him having red hair and fans are speculating that is for his return in study group season 2 since the character had red hair too. I think that we have to wait until the official announcement if also the old cast will comeback or not . they might announce it later , we don’t know . but let’s wait . I also think they’ll all join