Phoenix area schools all are required to provide busses for junior high and high school kids that live more than 2.5 miles away. That is generally a walkable distance, even in the May and August heat. I know, as from middle school on I did exactly that (walked or biked ~2 miles to and from school in the Phoenix metro).

For longer distances it’s really not a big deal to bike to school. No one needs to be dropped off by their parents, unless they live out of district and chose to go to a school other than their local one. And even in that situation, many schools open a half hour or more early, where kids can be in the library or cafeteria well before class starts.