Hi shorehamite,
It might or not be an actual breach of the General Data Protection Regulation or other legal or moral infringement.
I've got all my student's phone numbers in my phone. If your (ex) instructor rings his students up and invites them to move with him, you are obliged to send the new DTO Head of Training their records.
Poaching of customers has probably been going on since Roman times or even earlier. I know it's annoying especially if you've already fallen out with the instructor and find it galling that they are trying to take away your business.
HOWEVER, those customers they do manage to persuade away might think they have good reason to go. If so, perhaps they wouldn't be happy staying with you and you're better off without them. If, later on, they find out that the instructor who left you wasn't up to much and want to come back, welcome them with open arms with no recriminations. Read the parable of the Prodigal Son.
Going the GDPR route will only cost you money and a loss of reputation.
TOO