Assuming a spherical stay at home parent...
The single-income family is a thing of the past. This is no surprise to most people in the USA. Even if they're somehow able to pull it off themselves, they're well aware that their friends and neighbors do not have that luxury.
Unless, of course, you're a school system.
The model for school systems inherently assumes that one parent stays at home. (And if you're a single parent, it's worse.)
The rules may vary with location, but the trends are clear. If your child is sick, they need to stay home. If there's snow, they have to come home early. Or leave later. Or stay at home the whole day. If you need to meet with employees, expect to do it during regular business hours , even if you are also working during that time.
Daycare helps - especially with snow days - but many daycare centers only go up to age 12, and some of them won't watch your child if they have a fever. For example, I'm working Monday - it's going to be perhaps our busiest day in a week since so many other people have it off.
It's hard enough for me - and I'm in a white-collar job with a supervisor who is pretty understanding about these kinds of things. I can only imagine how much harder it would be if I was still in a food service job, or on a factory floor, or construction site.
I'm not sure what the best solution to this is. But the assumption that one adult is always available is not only wrong, but will make communication and cooperation between the school and home more difficult.
 As a side note, many teachers go out of their way to meet after regular business hours. Administrators and other employees who come in contact with your child - like bus drivers - are a different story entirely.