School tax rates in Nassau school districts will be higher than expected because of County Executive Edward Mangano's "mismanagement" of the property tax assessment system, his Democratic opponent Thomas Suozzi said Tuesday.

At a news conference in Mineola, Suozzi criticized Mangano for giving reductions to 87 percent of homeowners who challenged their assessments. When a home's assessment decreases, neighbors pay more in taxes if they don't challenge their assessments, or if their assessments remain the same or increase.

"Peoples' property taxes are going up because Ed Mangano broke the assessment system worse than it's ever been before," Suozzi said.

Mangano spokesman Brian Nevin accused Suozzi of "distortions" and said the administration has worked to settle grievances to curb the county's liability for successful assessment challenges. The administration and legislative Democrats feuded for months over Mangano's request to borrow millions of dollars to cover settlement costs.

Mangano has "made certain every resident was given the ability to correct [their] assessment before demanding payment, thus saving taxpayers $30 million annually and eliminating all future residential tax grievance debt," Nevin said.

Last year, school tax rates jumped an average of 11.7 percent countywide, fueled by falling home values and successful assessment challenges.

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Figures provided Tuesday by several Nassau school districts indicated that residents may see increases again this year, although they will be smaller. The increase will generally exceed the tax levy passed by school districts in their budgets this spring.

While 2013 countywide figures aren't yet available, Lorraine Deller, executive director of the Nassau-Suffolk School Boards Association, estimated that school tax rates will increase by an average of 5 percent. That reflects a combination of increases sought by the districts and the effect of tax challenges.

In Rockville Centre, the school tax rate for the average single-family home will increase by 9 percent -- including a 3.5 percent hike enacted by the district -- costing homeowners an additional $400, said Superintendent Bill Johnson.

Last year, the district's school tax rate increased by 10.5 percent, including a nearly 2 percent increase set by the district.

Officials said the school tax rate for single-family homes in Jericho, Great Neck and Roslyn will increase by a total of 8 percent. Herricks residents will see a 7 percent hike and Baldwin homeowners will get a 6 percent increase.

Last year, Jericho had an 11.9 percent increase, while the hike was 11.2 percent in Baldwin.

New tax bills will be sent out to Nassau residents this week.