Data released by Boston Public Schools on Monday show that students and families experienced huge bus delays at the start of the school year.

On Thursday, Sept. 5, the first day of school, only 34% of buses were on time for morning drop off — the worst the district has reported in nine years. On the first day of school last year, 61% of buses were on time.

“The first two days were just really bad,” said Lena Ismail, whose son is a student at Boston Latin Academy. “It was an hour late picking up on Thursday and an hour and 40 minutes late dropping off because the bus driver had the wrong route.”

Beyond being frustrating, a bus delay can also have a domino effect.

“I have a daughter at home. We have to get her to school too. And I have to be at work by 8 [a.m.]” Ismail said. “It creates a chaotic morning when things are delayed.”

A total of 632 Boston Public Schools buses transport about 22,000 students daily.

The district’s state improvement plan requires BPS buses to have an on-time arrival rate of 95% each month, and 99% of buses should arrive at school no later than 15 minutes after the bell rings.

The highest monthly average that BPS reached was 90% in March.

“The on-time rate between the first and the second day was a huge improvement,” Mayor Michelle Wu said in an interview with GBH’s Boston Public Radio on Tuesday.

BPS data showed that on-time bus performance did improve on Sept. 6 for both morning pick up and afternoon drop off. But arrival rates declined again on Monday, when only 57% of buses got students to school on time.

Transportation issues also come as the district implements Zum, a mobile app that allows parents to track buses in real time. Drivers can also use GPS navigation on tablets instead of referencing a route on paper.

Wu said that more than half of BPS families have already downloaded the app.

“It is a learning curve with new implementation of this new technology, and it will take a little bit for that to settle,” Wu said.

The mayor added that more families than usual also registered their children for bus transportation during the last weeks of summer, which also caused the delays.

Boston City Councilor Ed Flynn has also raised concerns about the issue and called for a hearing on the matter, referencing complaints from “many parents” about school bus delays or buses that never arrived to pick up students on those first few school days.

“While some students were eventually picked up, others had to make alternative arrangements to drop off their children when their buses did not come,” Flynn wrote in the order.

“There should be reliability and predictability in BPS transportation,” Flynn wrote, calling large wait times for students and families “unacceptable.”

Flynn also wrote that “BPS and BPS transportation should consider coordinating a dry-run with school bus drivers, parents and students before the first day of school” so that drivers can get familiar with the bus route and discuss any drop off or pick up issues with parents.

Boston Public Schools did not immediately respond to requests for comment.