From medicine and law to electricians and politicians. The difference is twofold, however: if you have a cruel doctor, you can either sever your relationship with him/her and go to another one, and/or complain to a board who can take away their license. Students don't have a choice: they are required to attend school, but they can't leave a "cruel" (in your example) or poorly performing teacher. They're stuck with them. For at least a year--a year in which they have lost out on learning a subject.
Let me be clear: I absolutely 100% support teachers having tenure: it's critical to their ability to express themselves freely and be secure in the positions. However, having witnessed a couple of true clunkers (really inept and unprofessional) teachers among the many excellent and highly professional ones my kids had when they were in school, I am of the opinion that the tenure system probably needs some tightening up. 18 months is far too short of a time to acquire tenure, and the criteria on which it is based are in many places weak or nonexistent. It's basically a question of whether you showed up for the 18 months.
My husband served for several years as chair of the tenure committee for the college at which he is a professor. I was overwhelmed at the amount of work all the members of that committee had to put in to reviewing each candidate—dozens of hours each in review and then dozens more in discussion and reports—and and how long and arduous the process was for the candidates, publishing and presenting lectures and research (the tenure often takes 7 years for a professor). And then it all had to go to outside reviewers as well, before passing on to administration. Once done, a professor's tenure is not fully iron-clad, either: if you screw up royally in some way, you can be fired immediately.
I'm not saying the process can or even should be as strict as that, but perhaps what we need here is a bit of tightening up on the review and granting of tenure, to make sure incompetent candidates don't make it through, and then another process on the other end that can swiftly and effectively remove teachers who may have gone off the deep end later on.
But we should definitely keep tenure for our teachers. Students, on the other hand, should have a right to an equal education. The system perhaps needs some tweaking.
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