Even if classrooms are ready and principals are prepared, the success of the mayor’s flagship universal pre-K initiative will soon rest in large part on the interactions between 53,000 four-year-olds and their teachers.

“On September 4, the ball gets passed to you, all over this city, to be the people to bring to life this noble idea,” Mayor Bill de Blasio told hundreds of pre-K teachers on Tuesday.

The teachers were gathered at Brooklyn College for a three-day training, just 16 days before classes in district schools begin for the year. In back-to-back speeches, de Blasio and Chancellor Carmen Fariña reminded teachers of their critical role in an effort that has high stakes for both students and the city.

“You’re going to get [students] on track, and they’re going to stay on track with the help of the other changes we’re making,” de Blasio said, mentioning the city’s parallel expansion of after-school programs.

Fariña, who spoke before the mayor, mixed encouragement with specific advice, telling teachers to get a sense of the kids’ natural pace and “move with that tempo.” She also added a dose of realism when she said, only half-jokingly, that teachers might need to take naps after work for the first few weeks.

Over 50,000 four-year-olds’ first foray into formal education also represents the de Blasio administration’s biggest effort to improve student achievement. The institute, offered three times in August to new and returning pre-K teachers, is the city’s most visible effort to make sure the teachers are ready.

The city estimates that about 1,000 teachers participated in this round of the free training, which is designed to familiarize them with Common Core-aligned standards for pre-K and with specific strategies for helping students adjust to a school setting for the first time.

Tha Mcbride, who will soon start her fourth year teaching pre-K in Brownsville, said she came to the institute hoping to find new strategies for working with her students.

“I’m still feeling everything out,” she said. “It’s refreshing to have support from the mayor.”

In his speech, de Blasio acknowledged that providing training and a detailed handbook is only one step toward the city’s goal of providing consistently high-quality pre-K. Since city officials first announced its pre-K expansion plan, they have promised to send teacher coaches to pre-K classrooms and add staff members to help monitor the programs.

“We don’t want to set in place high standards and walk away. We want to figure out with you how to reach that high level,” de Blasio said.