Tests will be done on the substance, but Wilkinsburg, Pa., police suspect the third-grader was passing around the drug Tuesday at Turner Elementary School.
Police say the bags had the words "trust me" stamped on them and would have a street value of about $1,000, if it is indeed heroin.
School district officials in Wilkinsburg, located 10 miles east of Pittsburgh, sent a letter to parents warning that some students may have had touched the substance.
The letter also asks them to discuss the matter with their children.
Police say the investigation is continuing. No charges have been filed.