PROVIDENCE, R.I. — Faced with soaring diesel fuel costs, school districts are forcing students to use the old-fashioned way to get to class: on their own two feet.

Many schools are eliminating or reducing bus service because fuel had jumped to $4.50 a gallon, 36% more than a year ago, and is busting budgets.

In California, districts are eliminating busing for thousands of students. Districts in Washington state, Idaho, and Maryland and elsewhere are consolidating bus stops, canceling field trips, and forcing students to walk longer distances to school to control costs.

Worried parents in Massachusetts have called WalkBoston, a nonprofit group that promotes walking, asking for help after their communities cut back on busing.

Health advocates long have encouraged students to walk, stressing the fitness benefits. But school and transportation officials say they fear that abruptly reducing bus service could lower attendance rates, increase traffic congestion, or endanger students if they cannot walk on sidewalks and crosswalks.

“If you remove a school bus from the road, you’re adding 40 to 50 cars in the morning and in the afternoon,” said Bob Riley, spokesman for the American School Bus Council, which represents school transportation officials.

Major cuts loom in California, where schools are not required to provide transportation to campus. As a result, districts squeezed by fuel prices and fewer state dollars are trimming millions from transportation budgets.

The Capistrano United School District in Orange County, for example, has eliminated 44 of its 62 bus routes to save an estimated $3.5 million, a district spokeswoman, Julie Hatchel, said. The cuts will affect an estimated 5,000 students from kindergarten through high school.

Leaders in three communities served by the district have threatened lawsuits, saying school officials are ignoring traffic and pollution implications. While cutting bus service is unpopular, Ms. Hatchel said it is better than firing teachers and increasing class sizes.

“Our goal was to keep those cuts as far away from the classroom as possible,” she said.

Increased fuel costs are especially punishing on large, spread-out districts. The school board in Montgomery County, Md., covers the sprawling Washington beltway. It buses 96,000 children daily and burns about 3.3 million gallons of diesel annually. Each penny per gallon increase in the diesel price means an additional $33,000 in spending.

Seeking ways to contain fuel costs, the school board has authorized its superintendent to force students to walk farther to school. The current limits now stand at 1 mile for elementary school students and up to 2 miles for high schoolers.

“What if fuel should go up by a quarter in a period of a month or two?” a district spokeswoman, Kate Harrison, said. “We might have to have some emergency response to that.”

Small towns are feeling the pinch, too. Short on cash, school officials in Shirley, Mass., a small town about 40 miles northwest of Boston, are going to four buses from eight starting this school year. Students who live within 2 miles of school must walk, bike, or get a ride.

Parents in Shirley are worried about safety and seeking help from WalkBoston. Mary Day said her two sons will have to cross train tracks on their routes to school. To compound the problem, the town recently got rid of its crossing guards to save money.

As a single, working mother, Ms. Day said she can drop her children off at school in the morning but cannot pick them up. Her street runs parallel to train tracks and she fears her 9-year-old and 12-year-old sons will be tempted to take shortcuts by darting across the tracks outside the official crossings.

“I remember being a kid,” Ms. Day said. “Are you going to walk a half-mile down the street to cross in the appropriate way when you see a clear way right there?”

Her youngest son, Quincee, isn’t thrilled with the idea of walking, especially when the weather gets cold.

“I don’t really like it because it takes like 20 minutes to do it,” he said.