Most people do not know that their "occupation" is making money by getting the most they can for the skills they have. That occupation involves way more than merely plying your craft.

It is not up to your employer to decide what you are worth. It is up to you to demand what you could get elsewhere, including privately, taking into account your real or perceived ability to find private employment.

If you hadn't taken the "underpaid" job, would you have been offered the better one? Why didn't you find out what you could charge privately and then advertise your services? Maybe you only got 14% because your job exposed you to private offers without you having to find one. That's not a bad strategy if you lack the skill to find private engagements on your own. By getting you work, your employer acted as your advertising agent and publisher of your advertising to a perfectly targeted audience. That's way more than "some materials." What's that worth to someone who, for whatever reason, chose not to just hang up a shingle?

Why wouldn't you "steal" his students? Did you contract not to steal them? The clients offered to be stolen. If your employer doesn't pay you enough to make stealing students worth your while, or worth the students' money, who is he to complain when you accept an offer from one of them?

People who lack entrepreneurial talent suffer from a Dunning-Kruger-like belief that there is no such thing as entrepreneurial talent. To them, entrepreneurship is essentially a character flaw. It never occurs to anti-capitalist workers that their employers "supply them" with end-users that the workers could not otherwise find, much less organize to serve. Viewed from the right perspective, employees "hire" an employer to tell them what to do, and they pay that employer on a contingent basis. Now THAT is exploitation.