Teachers' dues from teachers' salaries. Salaries whose levels are based on things, like the required dues, and are funded from tax dollars.
I don’t know about TX, but where I live in VA, teachers are paid by the County which receives very little federal funding (2.8% of the annual budget). Most of the school budget is from property taxes.
Nonetheless, the salaries are theirs, the teachers, and most of the time it’s been earned. That is part of our social compact; that we are taxed for certain purposes, including schools. I don’t have a problem with that so long as the teachers are doing their jobs.
As only one example of the kinds of teachers my kids had, one of their class advisors was eligible for retirement the year before my kid was to graduate, receiving something like 94% of her average salary for the previous 3 years (nice benefit package!) However, that teacher chose to stay and finish her time and commitment to that class. So, she worked that last year for 6% of her salary, forfeiting the other 94%. I don’t think you’d find her picketing anywhere.