BELL, Calif. — When thieves broke into the high school music room here this week, they cut through the bolts on all the storage lockers and ripped two doors off their frames. But they didn’t touch the computer or the projector or even the trumpets.

“It was strictly a tuba raid,” said Rolph Janssen, an assistant principal.

Bell High School is only the most recent victim in a string of tuba thefts from music departments. In the last few months, dozens of brass sousaphones — tubas often used in marching bands — were taken from schools in Southern California.

Though the police have not made any arrests, music teachers say the thefts are motivated by the growing popularity of banda, a traditional Mexican music form in which tubas play a dominant role.