I posted this analogy elsewhere today, but it might illustrate the issue:
Imagine a school day as a typical 6 hour InterCity train ride.
Every seat is full, in fact over-full. Some toilets are working, others are not. There is a single buffet. The windows only open marginally. Occasionally, a member of staff wanders through to pick up rubbish, empty bins and perhaps run a cloth over a spillage.
At the start of the train ride, everyone - all kinds of people, from smart businessmen to those whose behaviour might normally give cause for concern (the odd group of football fans, a hen party, some with very dubious hygiene habits etc etc) - join the train from a crowded platform of family members seeing them off.
While on the train, to be like a primary school, every carriage has to stay where they are EXCEPT for a couple of random 'in the middle of the countryside' stops, where groups from every carriage rush around madly, jumping on each other, huddling in groups etc. Quite a few of the rowdier groups do find the sitting in one place quite tricky, and may wander around their carriage.
To be like a secondary school, every hour, every person on the train must find another seat in another carriage, with the usual squash in doorways and along the corridor. Some coaches have specialised equipment, which different groups use all journey.
In every carriage, there is one person who is 'in charge' and has to make certain that every person sitting in the carriage receives and understands a particular message, and can repeat it perfectly. In the carriages replicating 'primary' classrooms, this person in charge has to repeat the message personally to at least half the carriage, by walking through it and speaking directly to them, close to.
At the end of the journey, the whole train disembarks, again to a crowded platform of family members, including those who are ill or frail or medically vulnerable, or who share houses with those in that category.
This journey is repeated daily, for all the same passengers.
For this to be reasonably safe, especially given recent articles on patterns of infection such as this one you really want as few people as possible to be on the platform / joining the train while carrying the virus. This will only come about IF the numbers of people in the community infected is very low, by keeping the R number below 1 for a long period.