Police say the suspect, Eric Duong, worked as an extracurricular activities informal volunteer at Gladstone High in Covina.
They said the parents of a student at Gladstone High in Azusa told police Monday that they believed their child had an inappropriate relationship with the volunteer.
Duong also worked with El Modena High School in Orange, Tustin High School and West Covina High. He's being held on $50,000 bail.
The Associated Press contributed to this story