Interestingly my piano teacher that I had throughout my teens did it this way : She never "raised " her students'
rates. But upon taking on new students, she would charge these a rate based on the current value, accounting for cost of living increases. So in actuality, students who had been taking lessons with her were still paying, for example, $15, while new students starting today will pay 18$. She kept track carefully in a record book of who was who and how much they paid, and she never asked for the "wrong" fee from me. I did not know that she never raised a student's fees until I had taken from her for 2 years, it came up, incidentally, in con versation
While this may seem somehow not fair at first , it is fair and does make sense after thinking about it this way.: She was rewarding established students for their loyalty to her by never raising their rates unless the quit and tried later to start up again.
Of course all of this would only benefit you as a teacher if you teach full-time, and therefore have enough "turn-over" to get a raise in your actuial total teaching income...
Just a thought