Oh, I would definitely advise not to tell one's boss that you're about to be their competitor. I'm fortunate enough that my boss was enough of a spacy dumbass that I genuinely don't think he put it together that me leaving and my clients leaving were connected. For anyone planning something similar: part of a tutor's job is to build a connection with the students and parents they're working with, so it's not unusual for a client to walk when their tutor isn't available. People aren't coming to tutoring because they love your boss.