walking school bus
n. A group of children who walk to school together under the supervision of one or more adults.
Districts in Hunterdon County are among thousands nationwide doing their part each year by creating "walking school buses" to raise awareness of the issue.

An adult or group of adults begins walking along a designated route, making "bus stops" along the way to pick up other children on their way to classes. …

Last year, more than 700 county students took "walking school bus" routes to school, accompanied by friends, parents and faculty members.
—“Leave the car (and bus) behind and walk to school,” NJ.com, September 15, 2015
Participants say the walking school bus has built a greater sense of community. Parents, teachers and volunteers — all on foot — pick up students along a designated route on their way to school. Firefighters, police officers, even Sparky — mascot of the Somerset Patriots baseball team — participate on occasion. As they walk to school in sun, snow and rain, the bus continues to grow.
—Sarah Esteves, “Getting to school on a foot-powered bus,” CNN, October 08, 2014
In a community where 53 percent of the 800 elementary schoolchildren are chronically absent or tardy, the walking school bus is a community project to get these kids on the road to a more secure future.
—Lois Kazakoff, “Walking 'school bus' helps put kids on course,” San Francisco Chronicle, November 29, 2013
1996 (earliest)
Thinking along similar lines, members of the Greenest City Project in Toronto, Ontario, have developed the Safe Routes to School Program, which includes a "walking school bus": parent "bus drivers" take turns walking their own and neighbors' kids to school.
Turning Wheel: Journal of the Buddhist Peace Fellowship, July 01, 1996
October 7, 2015 is International Walk to School Day. Watch out for the walking school buses on your commute tomorrow morning.