This came about because teachers and their unions demanded it. Those of us who objected to it at the time were called mass murderers. Teachers demanded priority access to the vaccine early in the rollout, and they got it, because everyone wanted kids back in school. That was in December and January, and the teachers still refused to go back that year. Many teachers were rightly disgusted by that union decision, but they can't do anything about it as individuals.
What's even sadder is that because of union histrionics, many parents are unwilling to send their children to in-person classes even when schools open back up. The unions incessantly claimed that their refusal to teach classes in person was for the safety of the students, because covid was unacceptably dangerous to children. It was clear fairly early on that this wasn't true, but they stuck to that talking point because emphasizing the risks to teachers invites responses like "If you believe your job is unacceptably dangerous, you don't have to keep doing it, but we aren't going to keep paying you." Unions leaders knew all along that this was self-serving nonsense, but many parents believed them.
The problem here is that if there are 30 unvaccinated people in a room, one of them (the teacher) getting vaccinated doesn't do much to decrease the risk to the other 29, because they can all still catch it from each other (and probably still from the teacher, at the rate things are going, but that's a separate conversation). So someone who believed the teachers when they said in-person classes were too dangerous for students a year ago isn't going to send their child to in-person classes now, because in-person classes are no less dangerous now than they were then. And they're understandably confused that some of the same people are now saying it's safe for children, when the only that's changed is that teachers - not children under 12 - have been vaccinated. If the union mouthpieces had been honest about their rationale for refusing to teach in person, this problem wouldn't exist. But because they chose to mislead and sometimes outright lie to the public, huge numbers of children will needlessly miss out on in-person instruction because their well-intentioned parents believe it's too dangerous for even a perfectly healthy child.