D.C. School Superintendent Arlene Ackerman has asked parents of all students in the city's public schools to bring proof of residency to their schools before classes begin Sept. 1 -- or risk having their children kicked out of the school system.

After years of uncertainty about how many nonresident students attend the city's 146 schools, the new superintendent and her staff are mounting a vigorous effort to find those students and collect thousands of dollars in tuition from their families.

"It is important for us to get an accurate count of students and nonresidents, so we know exactly how to spend the money we have," Ackerman said yesterday.

Annual tuition for nonresidents ranges from $4,703 for all-day kindergarten classes to $6,420 for elementary school. Junior high costs $6,162 a year, while senior high is $5,615. Special education costs are even higher. Visually impaired students, for example, are charged $31,754.

Letters outlining the requirements have been sent to parents and legal guardians. If they can't provide documents proving residency by Sept. 15, the students will be asked to leave. Some exemptions will be granted if proof of residency can't be established, but these will be done on a case-by-case basis. Officials have declined to say what criteria they will use.

The D.C. financial control board approved regulations earlier this year requiring parents to provide copies of one of the following: proof of payment of a D.C. personal income tax; a current withholding statement; current official documentation of financial assistance from the D.C. government; a valid D.C. driver's license or nondriver's identification.

In addition, families must provide two of the following: a vehicle registration showing the parent's or guardian's name; a title to residential property in the District; an unexpired lease agreement receipt for a period within two months preceding consideration of residency; a voter registration card; one or more utility bills; paid receipts or canceled checks from a period within the last two months.

The residency crackdown is one of many initiatives undertaken by Ackerman since she took charge of the troubled school system in May. Two long-standing problems have been an inability to come up with an accurate student count, and difficulty identifying nonresident students.

School officials say that despite the system's troubles, many District schools are as good as any in the suburbs. The District school system is one of few in the area that offer free all-day prekindergarten and kindergarten, and often free or low-cost after-school programs. Also, many parents who have moved out of the District still work there and either drop their children off on their way to work or have relatives in the city who baby-sit after school.

According to an audit of last year's enrollment count, about 30,000 of the school system's 77,111 students never provided proof of residency.

Early this year, Rich Wenning, then director of educational accountability, said there were up to 7,000 nonresidents at city schools who were not paying tuition. A 1996 report by the D.C. Council's education committee said there could be as many as 12,000 nonresidents, but others, such as demographer George Grier, suspect the number is far lower.

Ellen Opper-Weiner, a D.C. resident long involved in the issue, said the new rules are an improvement but aren't as tight as she would like. And unless officials enforce them, she said, nothing will change.

Sidney Yeldell, executive assistant to Ackerman, said 77,000 letters in five languages were sent to parents. Ackerman also ordered each school to designate a "registrar" to make a list of students who do not provide proof of residency. Some education activists say they fear that illegal immigrants and others without residency proof may keep their children home. Others predict that schools with low enrollments may not identify nonresidents because schools this year are funded based on per-pupil attendance.

It is unclear whether all parents received the letters. Some with children who attend private special-education schools paid for by the school system said they never got them. And some public charter schools opening next month may not be required to seek proof of residency.

The 10 schools chartered this year by the D.C. Public Charter Board are required to follow procedures comparable to the school system, executive director Nelson Smith said. But nine schools chartered by the D.C. Board of Education have nothing in their charters on the issue, said board member Benjamin Bonham (Ward 6).