Vaccines don't absolutely prevent infection. By the time the vaccines were out, we were already in a second wave with Delta which dramatically reduced the efficacy. Why should teachers, or anyone for that matter, be willing to risk their health? As a society, we treat too many people and jobs as literally expendable.
You also don't address how children can be carriers even if covid doesn't affect them directly. Child vaccines only recently became available, which reduces the infection possibility and therefore the ability to pass it on. By that time , omicron was a thing and has mutated enough that the vaccines aren't as effective to curtail spread.
The flu is significantly less spreadable/fatal than covid. People bringing this comparison up in 2022 are either wilfully ignorant or disingenuous.
Studies on long term risk to children have just started (1) so numbers are unavailable, but covid has already shown to have long term effects in some children as per my link in the other post
1. https://www.nih.gov/news-events/news-releases/long-term-stud...
All these anti lockdown comments tend to be the same, ignoring network effect and falsely trivializing the risk factor. Comparing to the flu is just ridiculous, and has been debunked over and over and over.