Sorry, no reports to date place the AR-15 in the school. NONE!
Use your preferred search engine with a phrase or two such as "lanza shot with bushmaster." Or "lanza shot with long rifle."
You'll find it's been three days since Chief Medical Examiner Dr. H Wayne Carver and the police identified the AR-15 as the weapon that Lanza used to kill the victims, and the Sig to kill himself.