The Oakland Unified School District is warning the public about alleged scam calls that attempt to extort money from parents by making it seem like their children have been kidnapped.

Some families have reportedly received phone calls from strangers who claimed to have kidnapped their child, saying they were holding them for ransom, the school district said in a statement this week.

The calls were allegedly made during the school day while students were in class. The caller would even use a child’s voice in the background saying something like, “please don’t let them hurt me.” The caller would then demand money in exchange for the child being released.

The school district said the attempted scam happened to the family of one student once this week, once earlier this month, and a few other times this fall. In every case, the child in question was in school and safe.

“Every day, our focus is the safety and security of everyone in our community, especially our students,” OUSD chief academic officer Sondra Aguilera said in a statement Wednesday. “Unfortunately, this scam shows the heartlessness of some scammers and the lengths to which they will go to squeeze money from unsuspecting people.”

OUSD officials assured parents that students are safe and protected in district schools, and that they will be notified if their children are absent from class.

Parents are also advised to write down the phone number of anyone claiming to have their child, obtain as much information about the caller as they can, and report the suspected scammers immediately to Oakland police. Parents should then call their child’s school to confirm that they are in class and safe, the school district added.