The Board of Education for the nation's biggest school system voted to require its half-million elementary-school pupils to wear uniforms.

Under the unanimous vote Wednesday, pupils must comply with a dress code starting in 1999 unless their school decides against uniforms or their parents are granted exemptions."This policy is important to diminish peer pressure, promote school unity and promote school pride," board president William C. Thompson Jr. said at the meeting.

But the New York Civil Liberties Union promised to sue if any student is coerced into wearing a uniform or punished for not wearing one.

Norman Siegel, the organization's executive director, said 20 families have approached him so far about fighting the recommendation.

The policy, which will go into effect in September 1999, will cover 670 schools, allowing each to decide the style and color of the uniform. Parents and pupils will help design them. High schools are not affected.

Board member Ninfa Segarra said the policy will help children "focus on their studies and not worry about an incident based on their clothing," although fellow board member Luis O. Reyes said he will choose an exemption from the rule for his son.

Mayor Rudolph Giuliani supported the rule, but parents and children were split on the rule.

"I like it," said Alicia Drygulski outside of Manhattan's Public School 59, where she was picking up her third-grader. "There'll be no competition."

Her daughter, Jennie, agreed. "I think it's a good idea because you wouldn't have to look at your clothes" every day to decide what to wear, she said.

But parent Ted Mejia, a former public school teacher, hates the idea.

"This is terribly condescending, to tell the parents you don't have the skills as a parent to make these decisions," he said.