Police spokeswoman Monica Munoz said students obtained a copy of San Ysidro High School's master key and snuck onto campus early Tuesday.
She says they were stacking desks and chairs in the hallway and putting furniture on a field in the shape of "09."
Munoz says their activity triggered motion sensors, which prompted the school's alarm company to call police.
Police cited the students who were 18 for trespassing, and those under 18 for curfew violation. They were ordered to return the furniture to their original locations.
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