The teacher is well liked by parents and students, and many say they hope this is all a misunderstanding. But some say they are glad he's on administrative leave during the investigation into the allegation.
In August, while his young son visited a doctor, the 41-year-old allegedly exposed himself in the reception area of a medical clinic.
"Once his son went in to see the doctor, he struck up a conversation with the receptionist and he made some mention that he likes to shock people," said Jim Amormino of the Orange County Sheriff's Department.
Authorities allege he talked about the annual event where hundreds of people moon passing Amtrak trains in Laguna Niguel. Then, investigators allege he pulled down his pants and exposed himself to the receptionist.
"He certainly did shock her but he also frightened her," said Amormino.
"It's kind of weird because he's a teacher," said Jaxen Gerrits, a student at Serrano Intermediate School.
"My daughter has class with him and after hearing what I've just heard, it kind of frightens me and scares me," said Noor Tahmass, a parent.
Other parents and students find it all hard to believe.
"He's a wonderful music teacher, all the kids love him, they love band, he's only positive with all the kids," said Rene Haas, another parent.
"He's really nice and very funny," described student Lauren August.
"We kind of have a petition on the board, we all wrote like, 'please let Mr. Petrash back,' and 'you rock Mr. Petrash,'" said student Flip Pejcinovic.
Petrash, a husband and father of two, is now facing a misdemeanor charge of indecent exposure.
"It makes me sick to your stomach to believe that someone like that is in a public trust position with our children. It's disgusting if the allegations are true," said Lake Forest resident Cameron Knauerhaze.
Petrash has been released after posting $10,000 bail.
School officials would not comment.