CHEHALIS, Wash. — Students on a field trip to a volcano on Monday morning experienced another kind of eruption: pepper spray on the bus.

The school bus was en route from Tumwater, Wash., to Mount St. Helens. After a student discharged pepper spray, the 28-year-old bus driver began to have trouble breathing but was able to pull the bus over safely just south of Chehalis "before she became overwhelmed," according to the Washington State Patrol.

Medical personnel and troopers responded to the scene and treated five of the 46 students for exposure. The bus driver was transported to a hospital for additional treatment.

Troopers found that a 13-year-old student discharged the pepper spray before giving the container to a 12-year-old student to hide it. As troopers were speaking with the 13-year-old, the second student passed off the pepper spray to another 12-year-old.

The three students were transported back to Tumwater and released to school officials. The district sent a second bus to pick up the students who were able to continue on the field trip.

Washington State Patrol officials said that the incident remains under investigation.