Luke saw that Grogu is already emotionally compromised by attachment, a thing that didn't work out too well for his own father. he remarked during training that it didn't feel like Grogu's heart was really in it, so he's probably worried that one day Mando will get kidnapped and tortured by some gang and Grogu will find out and go on a bloody rampage.
I don't remember Luke saying he'd be kicked out of Jedi school forever if he choose the chainmail. it's just a nice convenient sabbatical so that Grogu doesn't get squished by angry teenage Kylo.
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