With lawmakers warning about the possibility of mass confusion in the city’s schools and with SAT scores having plummeted to their lowest levels since 2003, Mayor Bloomberg could be facing his worst case of back-to-school jitters yet.

Six days before classes open, hundreds of New York City students do not know where they will attend school, many teachers do not yet have complete class rosters, and one high school is out a principal after he was removed days ago, to teachers’ astonishment.

In an interview at The New York Sun’s offices this week, Rep. Yvette Clarke, a Democrat representing Brooklyn, said she has “serious concerns” about this year’s school enrollment, saying her office has been flooded with calls from constituents nervous about where their children will start school Tuesday. “School is less than two weeks away, and they don’t know where they’re going to go,” she said.

“Expect chaos,” Council Member Robert Jackson, the chairman of the council’s Education Committee, said. “Expect the worst and hope for the best,” he said, summarizing his advice for the schools chancellor, Joel Klein.

Enrollment has often been a struggle in the city’s public schools, whose 1.1 million students are marked by higher transience rates than in most other municipalities, yet experts say this year the ordinary hiccups will be magnified by widespread confusion about how to address them, following Mr. Bloomberg’s latest restructuring of the school bureaucracy.

The shake-up abolished the system’s 10 regions, which were used to determine where teachers, principals, and parents could send questions and seek support. Schools are now grouped into community school districts, as they were when Mr. Bloomberg took over the schools in 2002.

When investigators at the office of the city’s public advocate called the new community districts this month, they got responses to one-third of 100 phone calls, Public Advocate Betsy Gotbaum announced yesterday, a record she condemned. After one office in Brooklyn returned no phone calls at all, investigators knocked on its door and found that it had not yet been staffed, a spokesman for Ms. Gotbaum, John Collins, said.

Ms. Gotbaum yesterday also announced she has opened a private hotline of her own to help parents sort out the new Department of Education hierarchy. The toll-free phone line was already ringing yesterday, Mr. Collins said.

A spokesman for the Department of Education, David Cantor, said back-to-school concerns are overblown. “People predict mass chaos every year, and in some respects, that’s a good thing, because it makes everyone more vigilant,” he said. “But we have a more responsive family engagement office than we’ve ever had, and we’re probably better-prepared generally than we’ve ever been.”

The president of the city teachers union, Randi Weingarten, said she has set her expectations high for Mr. Bloomberg and Mr. Klein. “After five years of being chancellor, one should know how to open schools,” she said.

Ms. Weingarten compared assurances that school will open smoothly to promises Mr. Klein made about new bus routes released this winter, which resulted in a debacle that Mr. Klein has said he regrets. “We gave them an early warning: If you cut the bus schedules like this, some students will be left out in the cold. But they said, ‘Don’t worry; we’ve got it covered.’ Now they’re saying the same thing,” she said.

Ms. Weingarten said the recent removal of the principal at the Jamaica High School in Queens, Jay Dickler, is an example of last-minute disruptions that make teaching difficult.

The school’s union representative, James Eterno, said he was preparing for a new school year when Mr. Dickler called him Monday night to say he had just been removed. “God knows how they make decisions down there,” Mr. Eterno said, referring to the Department of Education’s downtown headquarters.

A spokesman for the city principals union, Brian Gibbons, confirmed that Mr. Dickler is being reassigned but could not name his replacement.

Across the city, parents have been filling the auditoriums at schools designated as registration centers, waiting to learn where their children will go to school. A Manhattan mother, Luz Collado, waited at a center in Chelsea yesterday to find a school for her daughter, Melanie, who is on a waiting list for the Bard Early College High School, a list Ms. Collado said is 1,500 students long. Ms. Collado said she had tried to register her daughter for school in the summer, but was surprised to learn registration began only this week. “I don’t understand why they wait so close to when school starts,” she said.

A 15-year-old sophomore from Spanish Harlem, Dolly Camacho, was also waiting for news on a transfer she said she hoped would move her out of her old high school, where she said she did not feel safe after being hit in the face by a rival in February. But she said she did not yet know whether her requested transfer would go through, putting her in a limbo she said several of her friends also face.

She called the enrollment process “ridiculous.”

“Be happy a child is not falling through the cracks of the system and wanting to go to school,” she said. “If it’s a school I’m not comfortable with, then why can’t I transfer?”

Meanwhile, the Department of Education early yesterday announced that this year’s SAT math and reading scores are at their lowest since 2003, even as state tests show rising math and English proficiency.

Public school seniors who graduated in 2007 scored an average of 903 out of 1600 on the college entrance exam’s math and reading sections, down from 911 last year. The eight-point drop, the biggest in recent years, is double the decline in national math and reading scores, which fell to 1015 from 1019, though just one point below the average drop for public school students, a Department of Education spokesman, Andrew Jacob, said.

The discrepancy follows reports from local colleges that graduates of the city schools are poorly prepared for university work, particularly math classes, despite rising graduation rates touted by Mr. Bloomberg.

Mr. Jacob said the drop follows national trends.

A fellow at the Manhattan Institute, Sol Stern, said the scores are an indicator of Mr. Bloomberg’s poor progress in turning the public schools around. “This is social promotion,” he said. “Kids are graduating without the necessary academic skills to go to the next level.”