If my child got the chicken pox at school, and there were numerous children coming down with it over the course of a couple months, there is every chance that at any given point SOMEONE contagious may be walking around undetected at the school. At least with my child, I kept him covered up as much as possible, and warned anyone going near him (and who really does anyway, as they're rushing to get their children into class and then leaving) that he had them. At least he was a known risk that was contained in a pushchair, unlike the other children coming down with chicken pox that were walking all over the school.
Besides, my close friends weren't fussed about it. As one of them said, it's going around, if my child catches it, they catch it. None of my friends are pregnant. And nobody else got close enough to him to matter anyway. They were all rushing past to get their children into class and be off again.
I think on the whole if you're worried about catching them you have to take general precautions and just hope for the best. You can avoid every possibly contagious child at a school and get sneezed on by a toddler at the supermarket that is just starting to break out and then you could catch it anyway. You can't avoid all risk. That's life, unfortunately.
It's easy for someone to say "oh bring the child in late or pick them up early or late or whatever" but we all have lives, people have work schedules that are not easily adaptable, bus schedules to follow, car pooling, all sorts of other things they are dealing with as well. So sometimes they just have to bring the contagious child along and minimise their contact with anyone. And that, too, is life.