More than a dozen teenage high school students are under police investigation for a string of sexual assault offenses against two other Los Angeles high school girls, authorities said Friday.

Authorities arrested eight suspects at the high school on Friday while a ninth suspect was seized off campus. The tenth gave himself up on Friday afternoon. Police are still searching for the four others at large.

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Police detectives believed the sexual offenses happened on an intermittent basis - on and off - beginning in the last part of 2013 at Los Angeles' Venice High School, LA Police Commander Andrew Smith said.

Smith said several of the reported assaults were coercive sexual offenses while other cases were consensual but, he pointed out, "the victims were too young" to allow the acts.

He said incidents like these are a "terrible tragedy" and urged the public that ff there are more assault victims then he was asking them to come forward.

LA Unified School Superintendent Ramon Cortines said it was a painful day for the school district and Venice High School, adding that aid from crisis advisers would be at hand in the school to help students.

The former head of the school's association for parents and teachers said she was taken completely by surprise by the arrests, adding she was unaware of any complaints over sexual offenses at the school before the arrests.

The PTSA's Linda Patterson Salib said she has been part of the school for a long time but she has not heard of anything - whispered or in public - about the assaults. She was president of Venice High School's PTSA until February.

She said the police investigation comes as "quite a surprise" and it was not like she and the association knew there was a problem.