Students could not get into a California high school Friday after vandals plugged locks on all exterior gates and doors with super-strength glue.

"A teacher who always gets there early called me on my cell phone and said, 'Guess what?"' said Carlsbad High School principal Margaret Stanchi. "But I didn't guess this."

Roughly 100 doors were sealed shut sometime before the first staff arrived, Stanchi said.

"The locks are so small that all they'd have needed to do was put the point of the tube in and squeeze a little," she said. "It doesn't take much to stop you from putting your key in the lock."

Custodial staff quickly unsealed the lock on a building with 27 classrooms, but the rest of the school's 3,000 students were sent to sit in the gym until their classrooms were opened, one by one.

Surveillance video failed to give any clues to the prankster's identity, Stanchi said.

Police were investigating.