Kirk Waishwell, 38, admitted to using a hammer to smash the windows of 12 vehicles in the parking lot of Kensington Elementary School in La Grange in February 9.
He has no affiliation with the school, officials said.
PHOTOS: Car windows smashed at La Grange school
Waishwell also has to pay restitution for the damage.
He still faces additional misdemeanor damage and disorderly conduct charges.
He is due back in court September 28.