The demolition began Thursday. Newtown First Selectman Pat Llodra said the project will take several weeks.
"The process of demolition is incremental, staged precisely and executed carefully," she said. "There is no wrecking ball action; it is rather a piece-by-piece, section-by-section removal."
In May, city officials voted to raze the school and build a new one on the same property. The city has accepted a $50 million state grant for the project.
The new school is expected to open in 2016. Students have been attending classes in a neighboring town.
The gunman in last year's rampage shot his mother before going to the school and opening fire, killing 20 children and six adults. The gunman committed suicide as police arrived.
Contractors are being asked to eliminate nearly every trace of the school. The move is meant to protect the victims' privacy and keep people from taking parts of the building as souvenirs, according to Llodra.
Contractors also are required to sign confidentiality agreements to forbid public discussion of the site and the building.
The Associated Press contributed to this report.
