Scot Peterson did not go inside to face the gunman during the attack at Marjory Stoneman Douglas High School on February 14, which killed 17 people.
He resigned last week after being suspended without pay by Broward County Sheriff Scott Israel, who said he had been left “sick to my stomach”.
But Mr Peterson’s attorney, Joseph DiRuzzo, said it was “patently untrue” that the veteran officer acted like a coward.
He said his client believed the gunfire was coming from outside the school, and was simply following his training by taking cover and ordering a lockdown.
Mr DiRuzzo accused the sheriff of “jumping to a conclusion” about the officer’s actions during the attack.
His said in a statement: “Let there be no mistake, Mr Peterson wishes that he could have prevented the untimely passing of the 17 victims on that day, and his heart goes out to the families of the victims in their time of need.
“However, the allegations that Mr Peterson was a coward and that his performance, under the circumstances, failed to meet the standards of police officers are patently untrue.”
It comes after Sheriff Israel said the officer should have “went in, addressed the killer, killed the killer”.
And the president has since the initial police reaction to the shooting was “frankly disgusting”.
He said if he had been at the school, he would have ran in and faced the shooter “even if I didn’t have a weapon”.