I have worked at various institutes and at some, training was a mandatory requirement for the position and for others it was not. At one university, the chemistry courses were the most sought after courses by students and we partnered with the chemistry department to offer a course that would review the material with tutors and go over the best ways to help students learn the material. It was mandatory for all new and returning tutors. However, since it was worth 1 credit there were some exceptions like if it would affect a tutor's financial aid or something like that, or if there were other circumstances and the tutor had already been through the course several times.
I agree with Debbie that most jobs require mandatory training and as long as it's explained at the outset of hiring, I don't see why you can't make it a requirement. I would probably consider some possible alternatives and guidelines on what exceptions you would consider if you foresee it being an issue for some people. Offering it multiple days and times can also help minimize the occurrence of tutors being unavailable. For the course I mentioned above, the tutors enrolled in the course but there was not a day or time listed. Once everyone had enrolled, the instructor would poll all of the staff to come up with multiple days and times that would work for everyone, so we rarely encountered issues.
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