Some facts to counter all the finger pointing...
Vaccine uptake figures for this year are only available (on gov site) for the month of September, but they show September uptake among primary age children is in line with the same period last year (slightly higher in fact) with 32% this year vs 30% for the same period last year. So overall on track to have a similar figure for vaccine uptake among primary children as last year of 54%. The highest vaccine uptake has ever been in this group is 60% in 2019/20, and pre pandemic was only 44%. So this year's flu being worse likely has little to do with the number of children having their vaccines.
I havent checked all other groups in detail, but the ones published on the gov sites show other at risk groups (age 2&3, pregnant women, over 75s) are getting vaccinated at similar or higher rates than last year too.
Theres no figures that I can find easily for the number of teachers vaccinated as its not a specific group monitored by GPs/NHS/gov. Maybe that has had an impact of the number of teachers off sick, but they certainly can't be to blame for the severity of the flu and increased numbers alone.
The more likely issue here is the fact, already stated by a few posters, that the flu vaccine only covers specific strains of flu. The vaccine manufactures do their best to predict which strains of flu we will need protection against for this particular year but this is by no means an exact science and the virus can mutate/change to a new strains at any time.
Its most likely just a bad year for vaccine effectiveness and a particularly bad strain or strains that haven't been covered by the vaccine have managed to take hold. It happens. Its not great, but it'll pass, and next year will probably be much better because everyone that had it bad this year will have built some natural immunity on top of whats covered in the vaccine.
I know its nice to have someone to blame but in reality vaccine uptake hasn't changed significantly and is in fact higher than it was pre-pamdemic.