The only reason I can see to prioritise teachers is because we are not planning (currently) to vaccinate children at all, so will need to do everything else we possibly can to reduce transmission in schools.
The data shows quite clearly that - despite the noise we hear about it - they are at lower risk of both catching Covid and dying of it than many other occupational groups - restaurant staff, factory workers, taxi drivers all much higher (and not already getting vaccinated as the nurses, social workers and care workers are)
But as we work down the age groups, most occupations will start to be much safer, because increasing numbers of the people they mix with (general public or colleagues) will be vaccinated. That won't be the case for teachers until they licence a vaccine for children and get down the list to giving it to them.