Pro linebackers have to perform well, or else they lose their job.
The same can’t be said for union teachers.
And don’t forget, a “professional” just does it for money. An amateur does it for love. Just get someone to pay you for it, and you’re a professional. That’s it.
Last time I checked, not too many teachers have 50,000 or more people buying $50 tickets to watch them teach a class.