*** km 8/05 p. 1 par. 2 Part 12—Conducting Progressive Bible Studies ***
2 Once a Bible student qualifies to be an unbaptized publisher, he is likely already taking full advantage of the Theocratic Ministry School. The training he receives in preparing and presenting student assignments will help him to develop the teaching skills that are needed...
They don't have to be publishing yet to enroll on the school. I couldn't find any clarifying quotes on qualifications. However, I think it was a CO that explained that if they're doing something that would be considered a disfellowshipping offense, such a smoking, they shouldn't be on the platform, so they shouldn't be on the school.