In the school context, we have to remember that risk of infection is a function of probability that someone is infected, and then the distance, place and length of time you are in contact with that person, as well as protective measures such as PPE.
So a short encounter at a till might involve being in contact with lots of people (so a higher probability that one of those people is infected) but the short duration of the encounter lowers the risk of the infection being passed on.
A secondary teacher may come into contact with a very large number of people at a short distance for a short period of time (while passing along corridors etc) but will also spend, over a typical 5 lesson day, 1 hour with 150 different pupils, sharing the same air. So a smaller number of people that the till operator, but a much longer duration for possible infection.
A primary teacher will come into contact with a much smaller number of children, but that exposure is likely to involve closer contact and be for 6 hours each day.
A doctor has a higher probability of coming into contact with an infected person, even though the number of people they come into contact with is smaller than a teacher, and is likely to come closer to them, but has PPE.
If full classes of 30 return, still with no PPE and obviously with no social distancing in class, then it is this combination of risks that needs to be thought of, not just 'oh, teenagers have been meeting in big groups in parks' [open air].