Santa Cruz County Sheriff's deputies rushed to Aptos High after the student, a senior, was stabbed multiple times on the campus around 2:20 p.m.
A Calstar rescue helicopter landed on the campus, while medical crews arrived in ambulances.
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Authorities locked down the school and began searching for a suspect.
Two people were detained, according to the sheriff's office. Deputies later confirmed the suspects are students, one a freshman and the other a senior.
Both will be booked into juvenile hall.
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Aptos is about eight miles south of Santa Cruz.