(CBS) — An undiagnosed heart condition may have led to the death of a 13-year-old student at Thompson Middle School in St. Charles, officials say.

Cameron Pepera had just started seventh grade.

He was on the cross-country team. And on Tuesday afternoon at a meet at his school, he collapsed after running a mile.

“Normally it’s a two-mile course, but because of the heat we shortened it,” says Tim Loversky, principal at Thompson Middle School.

Students have started taking sticky notes and writing messages to Cameron and putting them on his locker.

“His parents came by school today and we walked mom and dad up to the locker and showed them the ways that Cameron had really impacted our student body in many, many special ways,” Loversky says.

“I thought it was neat that mom commented (that) she didn’t realize the impact that he had on our school.”

Loversky says counselors are helping students.