In North Texas, 25 school districts are taking part.
Athena Strand, who was the subject of an Amber Alert, went missing last week in rural Wise County, and her body was discovered on Friday night.
Tanner Lynn Horner, 31, is charged with capital murder and aggravated kidnapping.
Police said Horner confessed to abducting and killing Athena last week on Wednesday. Authorities said Horner was working as a contract FedEx driver and was delivering a package to the Strand family's home when the abduction allegedly happened.
Investigators have not revealed a motive yet. They said Horner did not know the family and has no criminal record.
SEE RELATED STORY: FedEx contract driver accused of taking and killing 7-year-old Athena Strand