The nation’s largest public school system is once again delaying in-person classes for most students. New York City Mayor Bill de Blasio on Thursday acknowledged the many obstacles for students to return to schools during the pandemic.

De Blasio’s announcement comes just four days before public schools were set to resume in-person classes. The mayor cited major staff shortages as the main reason for an additional delay, with a Thursday report from New York City’s Independent Budget Office estimating the city needing around 12,000 teachers or substitutes.

Union leaders have expressed additional concerns to be resolved, such as poor ventilation in older buildings.

Students will now return in phases. Students in pre-k and students with special needs will return Monday, while the start date will be Sept. 29 for elementary school students and Oct. 1 for middle and high school students. Remote learning will continue for all students as the phases take effect.

Around 42% of young students have chosen full-time virtual learning, which increased from 37% in the two weeks since the mayor and city’s teacher’s union originally pushed back the start date from Sept. 10 to Monday, USA Today reported.

The abrupt decision has left many educators, parents and school administrators frustrated and surprised, with many principals and teachers telling the New York Times that they only learned of the delay from the news.

“It is mid-September and there is still no plan on how to educate children,” said Natasha Capers, a public school parent in Brooklyn. Capers called the delay “a punch in the gut,” with others saying it’s a “slap in the face” and a “fiasco.”

New York City is not alone in the struggle to return to traditional learning, with 74% of the 100 largest school districts in the country choosing remote learning as their only instructional model.