Gardner was off campus eating lunch with the school narc at the time.
Gardner said he got a call from a custodian that he was needed in the school's back parking lot. A few minutes later, he encountered Harris, and the two exchanged gunfire. The exchange with Harris lasted for an extended period of time, during which Harris' gun jammed.
Gardner fired four rounds from his Sig Sauer .45 from a distance of 60-70 yards, none of which hit Harris who then retreated inside the school. Gardner did not pursue the shooter. It should also be noted that Gardner was not wearing his prescription glasses when he exchanged fire with Harris.
I CA in 1989, a madman used an SKS to kill kids on the playground. While he did enter school grounds, he could just as easily have done this from outside the property.