At a hearing Wednesday, Cruz, 23, began entering guilty pleas to 17 counts of murder and 17 counts of attempted murder in the Valentine's Day attack. His lawyer, David Wheeler, Broward County's chief assistant public defender, said last week Cruz intended to plead guilty to the charges.
Cruz was a 19-year-old expelled student with a history of mental health and behavioural issues at the time of the "cold, calculated and premeditated" killings, Broward County's State's Attorney Office in Broward County said in court documents.
The gun violence on Feb. 14, 2018, left 14 students and three staff dead and 17 others injured.
Because prosecutors have vowed to seek the death penalty, his change of plea from not guilty would open the penalty phase in which a jury would decide whether he should be sentenced to life in prison or death.
In Florida, juries determine whether to impose a death sentence. If prosecutors are not willing to drop the potential death penalty as part of any plea deal that may be struck with Cruz, then a jury would decide.
During last week's hearing, Cruz pleaded guilty to assault and battery of a law enforcement officer in a separate case in which he was accused of kicking, hitting and punching a sheriff's deputy as well as attempting to remove his Taser in a Nov 2018 jail attack.
Before accepting his plea, Broward Circuit Judge Elizabeth Scherer asked him if he understood the impact the assault case could have in the penalty phase of the murder case.
"Sir, I need to advise you that the state is going to be using this conviction in this case as evidence of an aggravating factor for purposes of arguing in favour of the death penalty. Do you understand that?" Scherer asked Cruz.
"Yes, Ma'am," Cruz responded.
Some of the teenagers who survived the deadly rampage formed "March for Our Lives", an organisation that called for gun control legislation such as a ban on assault-style rifles.
In March 2018, the group held a nationally televised march in Washington that sparked hundreds of similar rallies worldwide. Cruz was 18 when he legally purchased the AR-15 rifle used in the shooting from a licenced gun dealer.