As a Texas teacher's child, my answer is "what union"? TCTA (Texas Classroom Teachers' Association) appears to be toothless compared to the unions in other parts of the country: each year, my mom and every other teacher in her school district signed a blind contract that said that they could be sent to any school in the district, to teach any subject and grade level they were licensed for. In my mom's case, that could have been middle school elective Spanish (teach colors and numbers, sing fun songs, play Loteria, break up squabbling tweens), upper level high school Spanish (teach novels, mark essays, dealing with sophisticated language she hadn't since college) or 9th grade remedial US History (attempt to make surly teenagers remember important dates), the sort of change that could greatly expand her workload - hello, totally new lesson plans!
Not letting the state treat them that way would require looking at how their colleagues up north manage to get tenure and long-term contracts. But that would involve being in a true union.