BUT, school is always the same kids, and with less holidays, less chances of new seedings into the school groups, so whilst you might be meeting 30 kids, it's the same 30
Even in primary, with class 'bubbles', things are not as confined as you might expect.
Children might be taught in relatively fixed classes of 30 or so, but they will share toilets and lunch facilities with others. Equually, even more than during the holidays, they will go on to further activities and childcare - it's not unusual for a child to go to a breakfast club in school (across all age groups), then school, then an after school club (same setting but totally different set of children from the breakfast club, and different every night), then home, then on to e.g. Cubs or gymnastics or dance or swimming, sometimes in shared transport. The after school activities have been, in the main, admirable in their Covid efforts, but they are settings in which children from multiple schools and classes mix pretty closely, and it's quite normal for a child to attend 3 or 4 different activities over a week.