SAN JOSE -- Every Wednesday night, Oak Grove High School hosts a bingo night to raise money for its band program.

The school's fundraising effort took a hit this week when a masked man armed with a gun walked through an open gymnasium door and went straight to the cash box. The man demanded money and made off with an undisclosed amount, according to police and the school's band director.

Chris Moura, the school's band director, was not at the bingo event Wednesday but has spoken with several people who were in attendance. He said the robbery happened quickly, and most people "didn't know what happened until after it happened."

"He walked through a door, went over to the cash box and asked for the money with a gun," Moura said.

A school security guard began to chase after the robber but stopped when one of the parents shouted "Stop, he's got a gun."

Moura said the robber ran off with cash in the four-digit range. He said the bingo nights generate between $50,000 and $80,000 yearly. The San Jose Police Department's robbery unit will be conducting a follow-up investigation, according to Sgt. Heather Randol.

Thursday, Moura addressed the situation with his students.

"They're in shock," Moura said. —I told them it's just sad something like this happens that negatively affects great kids who are working as hard as they can to be as good as they can at something that they love."

The band program has a concert scheduled for Friday night at 7 p.m. in the school's main gym, with the program's two jazz bands, two concert bands and marching band all performing.

"We're going to celebrate great kids, great parents and great music," Moura said.