Grisham MS in NW Austin temporarily placed on lockdown

File photo of Grisham Middle School. (CBS Austin)

A Round Rock ISD middle school in Northwest Austin was temporarily placed on lockdown Thursday afternoon after an incident with a student, authorities say.

It happened for approximately 20 minutes at Grisham Middle School located just off Lake Creek Parkway.

The Williamson County Sheriff's Office says a 13-year-old student was caught with marijuana and then fled the school -- which then forced the lockdown. Deputies are still searching for the student.

A Round Rock ISD spokesperson says the student was being escorted to the office for disciplinary reasons when he became violent and ran off.

Further details were not immediately available.