More than 1,000 four-year-olds will have to wait to start their school careers after their pre-kindergarten sites were found to be unsafe or not yet ready to receive them.

Nine pre-K sites will not open this week because they failed to meet safety or health standards, city officials said Tuesday. The 265 children affected will be reassigned, since their programs are “unlikely to open this year” at all.

An additional 36 sites will have delayed starts, with most opening their doors on Monday, Sept. 8 after additional inspections, most to address ongoing construction.

Mayor Bill De Blasio and other officials indicated that the closures and delays were possible last week as he defended the city’s efforts to prepare sites for 50,000 children in response to a critical report from Comptroller Scott Stringer. Last year, 20,000 children attended full-day pre-K, and de Blasio has made the expansion the centerpiece of his first year in office.

“The last thing we want people to worry about is health and safety concerns,” Health Commissioner Mary Bassett said.

On Tuesday, the city said it has been in touch with the families of students affected by the closures and delays via phone, email and text messages, and that 83 students have been enrolled elsewhere so far. The departments of fire and buildings will do about 400 more inspections this week, adding to a total of about 6,000 inspections and walk-throughs.

The nine sites represent a tiny fraction of the 1,100 sites that will offer pre-K when school starts on Thursday.

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