THOMSON, Ill. (AP) — Tear gas drifting from a northern Illinois prison disrupted a middle school field trip to a farm causing symptoms in about 30 seventh-graders and school staff.

Authorities say the tear gas was being used during training at the U.S. penitentiary at Thomson on Thursday afternoon. The students were gathering potatoes in a field near the facility when their eyes and skin became irritated. Some had trouble breathing.

About two dozen students from West Carroll Middle School were treated at a nearby hospital.

West Carroll School District Superintendent Adam Brumbaugh released a statement Friday thanking the prison officials, local emergency crews and local police departments for making sure students were evaluated and treated.

He says families will receive follow-up information.