San Francisco Fire Department (SFFD) officials said the students are from James Lick Middle School in the central part of San Francisco. A couple of students were first showing symptoms of vomiting and nausea while one was found to have extreme symptoms.
The San Francisco Unified School District, where the high school is located, said in a statement that its staff members "are working with emergency personnel" to respond to the incident.
"At this time it appears as though some students have ingested a substance that is causing adverse reactions. There are 10 known victims who have been identified as being affected," it said.
It is unclear what the "unknown substance" is, but local media reports said it might be related to illicit drugs.
The SFFD said in a tweet that it had dispatched two Mass Casualty Transport Buses to the school for possible transportation of school kids from the impact area.