Investigators say the man approached the teenage girl from behind, grabbed her buttocks and then wrapped his arms around her in a hug of sorts.
When the victim screamed loudly several times, the attacker let her go and ran off.
Investigators describe the suspect as a Latino male in his 30s, about 5 feet 8 to 5 feet 10, with short hair and a stocky build.
In a statement, La Habra High School said administrators are working with law enforcement on the case and have made local schools, parents, students, and staff members aware of the attack.
Anyone with information about the case was urged to call La Habra police at (562) 905-9750.
City News Service contributed to this report.