That was always been a rule even 20 years ago. If you are designated as a car rider/walker you can’t just change it on a whim. Parents need to call the front office and change it. Yes being both is an option but the front office needs to know.
I can assume it’s to stop predators from telling kids if they ride their bike to school tomorrow they’ll give them money or something. Also if there’s no plan in place no one will know if you get injured or lost. Having an adult waiting for you or waiting on a call to tell them you made it home can be the difference between life and death sometimes.

The art of the deal truly doesn’t fall far from the tree