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DENVER — Health officials are investigating after between 25 and 30 students at a southwest Denver middle school got hives Wednesday.

Denver Public Schools spokesperson Will Jones said the students attend DSST: Henry Middle School in the Bear Valley neighborhood.

Most of the students are in eighth grade.

All after-school activities were canceled Wednesday.

The school nurse asked some of the affected students to stay home Thursday, Jones said.

It’s unknown what caused the outbreak.

DPS immediately contacted the Denver Department of Public Health and Environment, which went to the school Thursday to investigate.