First, it's not even close to impossible to fire bad teachers.
Do you want to fire bad teachers? Okay-- how will you identify them, and just how bad do they have to be in order to be fire-worthy? How many people have to agree that they are bad? Remember, in the Vergara case one student's example of a terrible teacher who didn't deserve tenure was a woman who was named Teacher of the Year in her district.
The "solution" proposed by reformy types is to define teacher effectiveness (teacher goodness or the lack thereof) by looking at how well students learned. But "how well students learned" really means "how well students scored on the big state tests."
Keep in mind that the Big State Tests often test only math and reading. Do you think you can judge the quality of an eleventh grade phys ed teacher by the tenth graders' scores on a reading test?
Also keep in mind that multiple studies show that scores on those tests correlate directly to the amount of poverty in a school. Poor, urban, and/or minority students will predictably score lower on the big state tests, which means whoever teaches them will automatically pull low evaluation scores, which means volunteering to teach in high-poverty schools is volunteering to have a low (and potentially fire-worthy) effectiveness score. What do you think would be the best way to recruit teachers for those jobs?
But aren't there Value Added Measure formulas that can correct for all that? The short answer is, no, there are not. There is not a shred of evidence that those formulas do what they're supposed to, and plenty of evidence that they do not.
Which means that, despite all the noise about tenure
These are also the schools in which teachers have to fight for their students, and fight hard, for everything from getting books for the classroom to speaking up about big-district policies that are unfair to the students, policies created and implemented by leaders who couldn't find their way to the school in question unless it was with a chauffeur and a GPS.
You build up any school by recruiting and retaining teachers, by building a staff that provides stability and security for the students there. You do not recruit teachers for high-poverty, low-achievement classroom jobs by saying, "Come work here. We'll chase you out the first time we get the chance, or the first time you annoy us." You recruit and retain teachers by saying, "We are investing in you for the long haul. We will work with you if you need help, and we will give you the support you need to do the job. We've got your back, and we're committed to you for the long haul. We promise that, barring actual malpractice, you'll keep this job as long as you wish, even when we find you annoying. We hope you'll think of this school as your home for decades to come."
You build up any school by committing to a relationship with the people who work there, not by letting them know that you'll only keep them around as long as they're useful to you. If you want to protect the civil rights of the poor and minority students in this country, you protect the rights of their teachers.