MANSFIELD -- Keith Azevedo's 11th- and 12th-graders knew he was into hands-on education when they signed up for his class in environmental issues. But the Mansfield High School students had no idea how dirty their hands would get.

Dirt happens when you're converting an old school bus to run on vegetable oil.

''I thought it was just something to get our minds going, get us thinking," 17-year-old Chris Mansour said of the classroom discussions about alternative fuels. ''I was kind of surprised when he actually had us trying to find a bus. I still can't believe I'm standing in this bus right now."

The 1991 vehicle is parked behind the high school. The first task was sanding off the ''s" and ''h" in its ''school bus" sign to christen it the ''cool bus." The students also have ground and brushed away its rust, painted its ceiling, and are ready to install a conversion kit so it will run on the throw-away grease from local restaurants.

Once it's complete, the bus, according to the plan, will be driven to local schools and colleges to spur interest in conservation and less-polluting energy sources.

The students' research indicates that they may be the first public high school class in the country to convert a school bus to run on vegetable oil.

The project has attracted volunteers from the community as well as other classes. Azevedo's wife, Meredith, teaches seventh grade in Mansfield, and her students helped clean the bus. A high school chemistry class concocted some biodiesel fuel, which will be used to get the bus started and warm up the veggie stuff so it can take over.

West Street Service Center Auto Body in Plainville is repainting the bus for free. The latest plan calls for a tricolor scheme -- black in the back, silver in the middle, and green in front. ''The idea is moving out of the dark ways of the fossil fuels into the green of alternatives," Azevedo said.

The School Committee lent the initial start-up money to obtain the bus -- $2,000, bargained down from the original $3,500 asking price on Don Brown Bus Sales' web page, www.buscrazy.net. And Arlen Shaw, of Shaw's Service Center, a Mansfield High School graduate with two sons at the school, offered to help get the bus to town.

Shaw and Azevedo took off at 5 a.m. Nov. 30 and found the bus parked in a huge lot next to the Hale Creek Correctional Facility in Johnstown, N.Y. Shaw slapped on repair tags, started it up, and drove the bus back, followed by Azevedo. The return trip took twice as long but was accomplished without a single breakdown.

''It's a great idea for the kids to do this," said Shaw, who said he hears gripes about the high price of gasoline every day at work. The students agree -- even though it has meant doing such things as sanding metal in the freezing cold.

''I love it a lot because we're not learning about global warming from a textbook; we're actually working on a solution," said senior Julianne O'Donnell.

''I didn't really think this could be done by kids," said 18-year-old senior Wesley Farinacci. ''It made me realize how much we can do."

Although their research could not turn up another high school class that has converted an old bus to run on vegetable oil, the students are well aware that their conversion project is not unique. It's estimated that more than 5,000 people in the United States power their vehicles with vegetable oil. (The concept dates back to 1900 when Rudolf Diesel developed his engine with the idea of running it on peanut oil.)

Dartmouth College's Ultimate Frisbee team, for example, converted a bus for a six-week, 10,000-mile, cross-country trip last summer that combined environmental proselytizing with Frisbee tournaments -- and a stop at the Hingham home of team member Kevin Garrity.

The New York City-based B.I.O Tour has logged more than 15,000 miles in its vegetable oil-powered bus, with a solar-powered sound system, preaching the benefits of alternative fuels, including the added benefit of barbecue-scented exhaust fumes.

And the transit system at the University of Massachusetts at Amherst is testing the practicality of using grease from the dining commons as fuel in a converted bus. Michael Garjian of Greasecar, the Easthampton company doing the conversion, said he expects the change will save thousands of dollars over the year, as well as reduce the amount of carbon dioxide and sulfur pollution spewed onto the campus.

The students are getting their conversion kit from Greasel Conversions Inc. in Missouri's Ozark Mountains, which ships out 10 to 15 kits a week and expected last year's gross sales to exceed $1 million. Another name in an industry that few know exists -- Deep Fried Rides in Tennessee.

Azevedo, 29, who has been teaching for four years and helps run a nonprofit health agency in Ghana, is pleased that his students have thrown themselves into the project. ''It's been such a great process to watch the students, to see their growth and the leadership that has developed."

He hopes the work going on in his classroom, and outside in the parking lot, will generate interest in alternative fuel research, as well as expand the 44 students' skills beyond their newly found ability to handle vehicular body work.

''The idea," Azevedo said, ''is to foster critical thinking, as well as problem-solving and analysis, and trying to meld that toward a social as well as environmental awareness."

It seems to be working.

''I didn't even know what biodiesel was until this project," said David Rumbel, a senior. ''It's been eye-opening. As I see gas prices shooting up, I see potential for this."

Johanna Seltz can be reached at seelenfam@verizon.net.

What do you think?

Do we need more educational innovations, such as the Mansfield High School project converting an old school bus to run on vegetable oil? Please e-mail your comments to globesouth@globe.com, with your name, hometown, and a daytime phone number (for verification only).