In some places, that's how speed limits are fixed.
In Texas, for example, they do a traffic study and set the limit to the 85th percentile speed.
https://www.txdot.gov/safety/driving-laws/speed-limits/speed...
This link claims this is for highways. Does TxDOT have a wider criteria for "highway" than the standard FHWA criteria? Under FHWA I would presume that school roads would be found on local or collector roads, not highways, but I'm not familiar with the streets mix nor the grant landscape of Texas cities.
Lets not take advice about anything in regards to road safety from the US, or specially Texas.
Doesn't that just cause speeds to creep up as people think it's okay to go a little over the limit?