Authorities say the thefts were reported over the last three weeks at Nicolas Junior High School, Richman Elementary School and Laguna Road School. The loss is estimated to be well over $30,000.
The suspects have been targeting air-conditioning units, as well as the surrounding copper wiring, Fullerton Police Sgt. Jeff Stuart said. The suspects broke into rooftop panels and cut the wiring in order to steal the copper.
Police have seen an increase in copper theft in recent years.
"Copper theft is huge in California because of the recycling value, the cost of copper," Stuart said.
As police search for suspects, parents and teachers are relieved to learn the air-conditioning units at all three schools are fixed and should be working as usual when classes begin next week.
Anyone with information about the thefts is urged to contact the Fullerton Police Department at (714) 738-6785, or Orange County Crime Stoppers at (855) TIP-OCCS.