A teacher was sent to the hospital following a prank involving what was reported as some type of smoke or stink bomb, according to officials.
The incident happened just after noon inside Montgomery Middle School.
San Diego police and San Diego Fire and Rescue were called to the school on Picador Boulevard.
The district said a student popped a bag with "foul-smelling contents." Firefighters picked up the contents to check and make sure the material was not some type of dangerous chemical.
Officials said a teacher was transported to Scripps Chula Vista. The district said the teacher was very nervous and panicked but was not harmed. She was sent to the hospital for observation.
Eight other patients with minor complaints were looked over by emergency crews.
The students were not sent home. Their parents were notified according to the district.
Teachers and school staffers are interviewing students to find out who was responsible.