About 20 million students in the United States ride to school each day on the familiar yellow bus. The vast majority of the nation’s roughly 490,000 school buses — which comprise the nation’s largest public-transportation fleet — are powered by diesel engines. “We’re poisoning our kids on the way to school,” said Jessica Keithan, cofounder and director of the Texas Electric School Bus Project, of the exhaust that inevitably infiltrates bus interiors and children’s lungs.

But that’s slowly beginning to change. Thanks to a slate of federal and state incentive programs, school districts all over the country are starting to swap out old diesel buses for new, zero-emissions electric-powered models.

This transition is reaching districts of all sizes and demographics, from Martinsville Independent School District in East Texas — which last year became the first in the country to go fully electric with four new buses — to Oakland Unified School District in California — which last month became the first large urban district to fully electrify its fleet, with 74 buses.

As the Environmental Protection Agency, through its $5 billion Clean School […]