SALT LAKE CITY — An on-call bus driver for the Salt Lake City School District was fired after he made a stop during a field trip that was not pre-approved by school officials.

Ryan Pleune stopped the bus at a rally for environmental activist Tim DeChristopher, who is currently on trial for placing fake bids on oil and gas leases.

Prior to the unscheduled stop, Pleune had driven approximately 50 students and several supervising teachers to a local playhouse to see "A Tale of Two Cities," The Salt Lake City Tribune reports. On the return trip, Pleune called a supervisor to say that he planned to make a detour, but did not specify the location or purpose.

During the rally, the students and teachers stayed on the bus while Pleune left the vehicle and took part in the demonstration, according to district spokesperson Jason Olsen. Pleune claims he got off the bus to ask demonstrators to sing, but did not take part in the demonstration.

Pleune said he believed the rally would be a "fun," "memorable" and "relevant" experience for the students, ABC 4 News reports.

District officials have not disclosed whether criminal charges will be filed against Pleune. The Salt Lake City Tribune reports that Pleune was told he was fired for violating district protocol by exiting the bus with kids on board.

The teachers on board the bus at the time of the incident could face disciplinary action, according to ABC 4 News.