I generally support teacher labor movements. However, I think it's important to consider the effects that even the smallest disruption. For example, last year in NC the teacher's groups organized a "Rally for Respect." What they did to bypass strike laws was they encouraged every teacher in the state to request unpaid personal day, which is normally rubberstamped by the principal as you only have a couple. When half the school wants leave, that would be when you stop signing them. But instead, to avoid that conflict, nearly all major districts cancelled school entirely, treating everyone taking leave as a foregone conclusion.
So, one extra teacher workday, barely even worth noting, right? Well, it just so happens that the day selected was the day of the AP Lang exam that year. By the time they saw fit to announce the non-strike, all of the exams had already been ordered and students and teachers had been planning for the normal date. Thanks to the non-strike, the state had to pay $45/exam to get late exams, and it could have caused some students to miss the exam if they had to take a conflicting exam late for a legitimate reason. Oddly enough, College Board wouldn't have charged if it were a real strike.