Actually few parents are curriculum specialists
or educators
or school administrators
or child/developmental psychologists.
Most of them have very few credentials for micro-managing their children's educational environment.
In many cases their (sometimes) well meaning meddling just muddies the waters and makes the job of educators more difficult.
Just because people are able to breed and squeeze offspring out of their vaginas doesn't mean they are automatic "experts" regarding all things child related.
It doesn't necessarily make them good (or even decent) parents.
And as for "parental rights" at school, even that isn't as cut and dry as it may sound.
On average these kiddos spend more waking hours with their educators, who are also their legal guardians during school hours, than with their own parents...and in many cases that's probably a GOOD thing.
Definition of in loco parentis in the Legal Dictionary by The Free Dictionary
legal-dictionary.thefreedictionary.com