Seven months after Mayor Bill de Blasio promised to extend the day by an hour at the city’s lowest-performing schools, the city is still negotiating key details with union leaders even as schools are left scrambling to schedule in the extra teaching time.

Since de Blasio unveiled his “Renewal” turnaround program last November, he has said all 94 struggling schools in the program will offer an additional hour of instruction, which research has shown can boost student attendance and academic performance. But with a few weeks left in the school year, principals are rushing to figure out how to convince teachers to work the extra hour next fall, which will be voluntary for staffers, and how to squeeze it in alongside mandatory teacher training and existing after-school programs.

The city Department of Education recently sent schools a three-page guidance memo to help with scheduling, but the head of the principals union made it clear Friday that he is unsatisfied.

The union “has requested on numerous occasions — but to no avail — educationally sound proposals from DOE regarding how extended learning time will be accomplished within our collective bargaining agreement,” Council of School Supervisors and Administrators President Ernest Logan said Friday in an email to members. He added that principals are expected to make a scheduling decision about the extra hour by June 10, “despite the fact that DOE has provided no clear guidance on this matter.”

Meanwhile, the city is still negotiating how much teachers will be paid for the extra instructional time and what the default extended schedule will be for Renewal schools that do not come up with their own, a United Federation of Teachers spokesperson said Friday.

The recent department memo lists some requirements for the extra time. It says Renewal schools must provide an extra hour of academics in addition to any programs they already offer before or after school, but that school staffers are not required to work the extra hour. Still, that academic time must be led by school staffers, though nonprofit employees may help out, the memo says. (A separate message to Renewal principals from Chancellor Carmen Fariña said they will get extra money in their budgets to pay teachers for that time, though administrators said they still have not been told how much.)

Schools have two basic ways to offer the extra instructional hour, according to the department memo.

The first approach is to fold it into the normal school day. Schools could do that by staggering teachers’ start times so that students’ school day lasts an hour longer, but no teacher works beyond the contractual 6 hours and 20 minutes.

The second route is to have teachers who choose to work an extra hour do so before or after the normal school day. But that option bumps into the 155 minutes per week reserved for teacher training, parent outreach, and “professional work” mandated by the teachers union contract ratified last year

The memo offers two workarounds: either have the entire staff do the mandated training and parent outreach before school so that the extra-hour teachers can stay after school, or have the extra-hour teachers and everyone else do their mandated work at different times.

All of those schedule changes would require the annual approval of at least 55 percent of teachers at those schools. And whatever option they choose could affect the time when students start school — and when parents have to get them there.

One Renewal school administrator said the ongoing negotiations and last-minute guidance had left the school with little clarity or time to make important scheduling decisions.

“There are so many unresolved questions,” the person said, and “not because we aren’t willing.”

Education department spokeswoman Devora Kaye did not respond to specific questions about the extra hour, including the concerns raised by the principals union president.

“In September, to best support these students, each Renewal School will provide an extra hour of instruction,” she said in a statement. “We are working with all partners to ensure each school has a model that works well and provides this extended learning time.”