A US high school teacher who hanged herself in her classroom has been found by shocked teenagers.

Students and another teacher found Jillian Jacobson, 31, at El Dorado High School in Placentia, California at around 8.40am on Monday morning, The Orange County Register reports.

"Paramedics performed life saving measures, however after several minutes the victim was declared deceased at the scene,” a Placentia Yorba Linda Unified School District spokesman said in a statement.

"The initial investigation indicates the cause of death to be suicide."

Ms Jacobson had arrived at the school early and locked herself in the classroom. Students in her first class of the day found her body after they had another teacher open the locked door.

Classes were cancelled at 11am.

The 31-year-old was a photography teacher who had been employed at El Dorado High School for eight years and was reportedly popular with students.

Students and friends of the teacher have posted tributes on Twitter.

"Thank you so much Mrs. Jacobson for teaching me how to take amazing photos, and also for being a great friend," one student wrote.

Ms Jacobson did not leave a note and what drove her to kill herself is unclear.

Crisis counsellors attended the school and have been meeting with the students who found Ms Jacobson's body and other teachers, school officials said.