Boston is down about 50 school bus drivers, officials said Thursday, creating a scramble to fill the job slots and work out routes a week before Boston Public Schools begins its first in-person school year since the start of the pandemic.

The school year begins Sept. 9 for grades 1-12. Kindergarten and Pre-K students begin the following Monday, Sept. 13.

The city is 40 to 60 bus drivers short, said BPS Superintendent Brenda Cassellius at a press conference alongside acting Mayor Kim Janey. Typically, Boston has anywhere between 700 to 720 drivers, Cassellius said, and is now finalizing a contract for about 650 bus drivers, Cassellius said. She added that BPS is still assessing how many students and families the shortage will impact.

Asked whether a pay bump incentive is part of the solution, Cassellius said BPS is “exploring all options at this point.”

“We will continue to communicate to our parents over the next couple of days and weeks as we continue to try to fill those positions,” the superintendent said. “We will also engage in some re-routing if they’re going to decline riding the yellow bus, which would then allow us to consolidate some routes.”

School bus driver paucity is a national trend playing out in many cities, but the BPS shortage triggered criticism from Janey’s political rivals Thursday.

“With hundreds of millions of dollars in federal aid coming to Boston, Acting Mayor Janey has no excuse for not having enough bus drivers on the first day of school,” said former economic development chief and fellow mayoral candidate John Barros in a statement. “If low wages are creating a hiring problem, we need to provide competitive incentives and wages. If BPS’s private bus contractor can’t get the job done, we need to work with them to fill the gap with alternatives."

Hours after receiving the Boston Globe’s endorsement for her mayoral bid, Andrea Campbell said in a statement that “parents deserve better” after the “immense” challenges of the past school year.

“City leaders must act quickly to fix this situation before school begins next week,” she wrote, “and with a transportation budget of over $130 million, there is no reason we can’t ensure every student has access to safe and reliable bus service.”

Experts say the bus driver shortage is being driven, in part, by COVID-19 safety concerns.

Responding to a question on whether the city’s vaccine mandate is playing into the shortage, as is the case in cities like Chicago, Janey said “there has been an interest” among some bus drivers to get more colleagues vaccinated.

“It is why we’re standing up vaccine clinics in the bus yards, as requested, so we will continue to work with them to get their team vaccinated,” she said.

It also yet unclear what portion of eligible BPS students have been fully vaccinated.

“In our schools [we’re] not sure what that exact number is,” Janey said, pointing to the city-wide vaccination data.

According to the city’s latest weekly vaccination report, 47% of children aged 12 to 15 have been fully vaccinated. For young adults aged 16-24, the portion stands at 55%. Children under the age of 12 are not yet widely vaccinated, as no vaccines have been green-lit for that age group.