The Dallas Morning News reported that Mesquite police found that Halpin, 52, pawned 270 items dating to last year, and began a case when $2,075 in cameras and a projector went missing from the school in May.
School administrators asked police not to pursue the matter, Mesquite police Lt. Steve Callarman said.
"Once we were made aware of the situation by the Police Department, we conducted our own internal investigation and we determined that at this time, all the items are accounted for," said Laura Jobe, a spokeswoman for the school district.
Halpin, who led Mesquite to the Class 5A title in 2001, declined to comment to the newspaper. He didn't immediately return a phone message Thursday from The Associated Press.
Halpin, who salary is $92,631, resigned Thursday as president of the Texas High School Coaches Association, said D.W. Rutledge, the group's president. His one-year term was to expire in July.
"We were shocked," Rutledge told The Associated Press. "I've got a mixed reaction. I've got a genuine care and concern for Steve as an individual."
Mesquite school administrators also did not promptly report the problem to state education officials as required, the newspaper reported. The Texas Education Agency said it plans to investigate.
A letter to the agency was sent June 5, about a month after police informed the school district of the evidence that tied Halpin to three Dallas pawnshops. All three had records showing that the district's property had come and gone multiple times from their businesses.
"They must report it," said Doug Phillips, TEA's director of investigations. "It's real serious."
Mesquite ISD Superintendent Linda Henrie said the district acted appropriately with its investigation.
"I don't want to be presented that we were trying to cover up something," Henrie said. "I do know that we reported it in what we thought was a timely manner."
During his 13 years as Mesquite's head coach, Halpin's teams went to the playoffs eight times. His son, Luke, is expected to be Mesquite's starting quarterback in the fall.