CHELMSFORD, Mass. — A Massachusetts school bus driver has been arrested after students about to depart on a field trip say they smelled marijuana smoke on the bus.

Chelmsford police say 63-year-old Ali Mahfuz was charged Tuesday with operating a motor vehicle while under the influence of drugs and reckless child endangerment. He was released on personal recognizance after pleading not guilty.

The Chelmsford High students said they smelled marijuana, and told a teacher. The principal then boarded the bus, also smelled pot, and had the teens disembark.

Police say Mahfuz had “red and glassy” eyes and his speech was “very slow and deliberate.” Police say Mahfuz was in possession of a small amount of marijuana and a glass pipe.

Mahfuz, of Nashua, New Hampshire, told the Boston Herald that he denies smoking marijuana.