CHICAGO (CBS) — Ambulances pulled up instead of buses outside a Back of the Yards neighborhood school Thursday, when five young students got sick after taking prescription drugs.

As CBS 2’s Marissa Parra reported, it all started just before 10 a.m. at Libby Elementary School, 5300 S. Loomis Blvd. That was when the first of the five boys had a “medical issue.”

Then, around 11:45 a.m., police said four of the student’s friends were “feeling funny.” So ambulances came for them.

The teens were taken to the University of Chicago’s Comer Children’s Hospital, where they were stabilized, the Fire Department said.

The Fire Department confirmed the boys took Xanax, which is used to treat anxiety and panic disorders.

The boys were all 13 or 14 years old, police said.

A Fire Department spokesperson said the teens might have overdosed on Xanax, but they have not determined for certain what happened.

Libby Elementary issued a letter to parents about the incident Thursday afternoon, saying “a small number of students ingested an unknown substance that they received from outside of the school.”

“We immediately notified the Chicago Police Department and the CPS Office of Safety and Security. We have contacted the parents of the students who we confirmed were impacted, and medical attention was provided to students if needed,” the letter said. “We are handling this situation in accordance with CPS policy.”

The boys were all expected to recover.