Arguably, if the district directed the students to go to an unsafe location or event, or otherwise inappropriate one, they could be liable for having sent the students someplace they should not. (E.g. dangerous neighborhoods; a museum show that is intended only for adults; a venue where alcohal or drug use, like certain concerts, is common; etc.). If that's not the case and the place to which the students are directed, like a local library or a public concert in town square, is appropriate and safe, the school/distriction should not be liable if injuries occur by events or actions beyond their control.