The Goenka schools provide a questionnaire to determine mental health and any medications the student will need. Each teacher is different in who they will let in, and all those schools are volunteer run. I can see that some teachers could be reckless with course admissions, but all teachers would have managed multiple courses before, and would know the difficulties for everyone if they let someone in who has a history of mental illness. It's not great situation for anyone if any student enters psychosis on a silent retreat, not the teacher/manager/other volunteers/other students.

Edit: Also it's tricky if your students are motivated enough to get in that they'll lie.

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