Very unusual exceptions? I'd say it's far more common than that - read this story (https://www.nytimes.com/interactive/2014/08/13/us/starbucks-...) and think about how often someone with this schedule is going to make it to parent-teacher meetings. Think about how well the 15% of adult Americans who are functionally illiterate[1] are going to use the internet or help even their elementary school student with homework. To pretend that we're just talking about parents who can't 'fully understand calculus' is just wishing away the problems.
[1] https://nces.ed.gov/surveys/international/reports/2014-piaac..., definition of 'below level 2' at https://nces.ed.gov/surveys/piaac/litproficiencylevel.asp