HOUSTON -- Two Houston middle school teachers were arrested on drug charges Thursday, KPRC Local 2 reported.

Police apprehended two teachers at M.C. Williams Middle School, 6100 Knox, in northwest Houston.

Investigators said drug-sniffing dogs found marijuana in the trunks of the teachers' vehicles in the school's faculty parking lot.

Police said an anonymous tip led to the arrests.

"It's absolutely disappointing, but by the same token, I believe you could take our drug dog into any employee parking lot and find two, if not more, hits on a car," said Norm Uhl, HISD's spokesman.

Six teachers and one custodian have been arrested within the past two weeks at five different HISD schools.

"We believe that the tips may be coming from other teachers who know of these individuals who are utilizing illegal drugs and, therefore, are coming forth," said HISD Police Chief Victor Mitchell.

Williams' principal sent an audio message to parents about the arrests.

Since the drugs were found in a drug-free zone, investigators said the charges were upgraded to a felony.

The teachers' names were not released.

The following HISD campuses have had employees arrested this year.

Woodson K-8
one teacher, one custodian
Worthing High School
one teacher
Wheatley High School
one teacher: Gina Michelle Thomas
Whittier Elementary
one teacher: Susan Lee Michael
Williams Middle School
two teachers