A school bus driver was arrested in Chelmsford for allegedly driving under the influence of marijuana after high school students about to leave on a field trip complained about a strange odor on board.

Ali Mahfuz, 63, of Nashua, was arrested Tuesday morning after students told a teacher about the smell, according to a news release from Chelmsford police.

“The teacher immediately notified the principal, who boarded the bus with other administrators and could clearly smell the odor of marijuana,’’ police said.

The Chelmsford High School students were taken off the bus and brought to a cafeteria, and Mahfuz was charged with operating under the influence of marijuana, negligent driving, and reckless endangerment.

Authorities said Mahfuz had driven a morning route for Greater Lowell Technical High School before arriving in Chelmsford. Officials in Tyngsborough, where the technical school is located, said they do not believe he used marijuana until after that morning route.

Mahfuz was working with North Reading Transportation Inc. Police said the company is cooperating with the investigation, and that a company supervisor came to Chelmsford with a new bus and driver to take the students on the field trip.

Chelmsford Police Chief James M. Spinney praised the Chelmsford students for reporting their suspicions.

“I cannot overstate the role played by the students today in ensuring their safety on the roads,’’ he said in a statement. “They knew something was not right, and they spoke up right away, alerting school administrators. I commend these students for their role in stopping a dangerous situation from unfolding.’’

Mahfuz appeared Tuesday in Lowell District Court, where he was released on his own recognizance after a not guilty plea was entered on his behalf. He returns to court April 26.

Andy Rosen can be reached at andrew.rosen@globe.com. Follow him on Twitter at @andyrosen.