Mario Macias's closet is filled with T-shirts revealing his affinity for cars, cartoons and such action movies as "2 Fast 2 Furious." He has plenty of jeans, too, at least one size too big so they cover the tops of his shoes and don't feel too tight.

But as he prepared to go back to school this week, he cast aside his favorites and opted for a pair of khakis, a blue polo shirt and brand-new black dress shoes.

It wasn't his choice. When the school year started in Prince George's County on Monday, Northwestern High School embarked on an experiment that most public high schools in the nation have refused to try -- making its nearly 3,000 students wear uniforms.

The Hyattsville school's students can choose from a list of approved attire that includes khaki, navy blue or black pants, skirts or dresses; white, khaki, blue or black shirts with no logos; and solid-colored shoes.

Necklines must remain at the collarbone, pants at the waist, skirts and dresses at or below the knee. Shirts must be button-down or polo-style and "not longer than the lowest part of the buttocks."

Gone are the staples of teenage fashion: baggy pants, jeans, cargo shorts and anything hooded, cropped, form-fitting, hip-hugging or midriff-baring.

"We want students to focus not on what's on their backs but in their books," said John Bois, a biology teacher who spearheaded the effort to adopt a dress code.

After President Bill Clinton endorsed uniforms as a way to curb school violence and foster learning in the 1990s, public school uniforms caught on at a rapid pace, especially in elementary and middle schools. Few public high schools have joined the trend, however, because enforcing the wearing of mandatory uniforms is difficult among older children, according to education experts.

Although most school systems in the Washington area do not require uniforms at any grade level, Prince George's has embraced the idea, with 93 of 197 county schools having either mandatory or voluntary uniform policies. One other high school in Prince George's requires uniforms -- Forestville Military Academy, one of the few public military schools in the nation.

Other area school systems, including those of Alexandria and Montgomery and Fairfax counties, do not have any high schools that require uniforms, officials said.

"I think there's just the common thought on the most part that teenagers . . . are a little older. They have ways of expressing themselves," said Michael Carr, spokesman for the Reston-based National Association of Secondary School Principals. "It's something people haven't really thought of doing, closing that off for teenagers."

The new clothing options for Northwestern High students do not seem like much of a choice to Isabelle Moreau, 15, a sophomore. "Everyone knows uniforms are dull and boring," she said.

As students switched classes Wednesday morning, the most colorful items on display were the occasional red backpack and dyed-green head of hair.

School officials said they are hoping to do more than change physical appearances. "We have TV and pop culture raising children and setting their mores and standards of behavior, " said William Ritter, Northwestern's former principal, who is a regional director for the school system. The result, said parent Marybeth Shea, is "a lot of cleavage and . . . low-rise pants. I just think we've gotten to a low, and school is not club time, school is not Saturday. School is work."

Whether uniforms can improve a school has stirred much debate. After becoming the first public school district to require uniforms in elementary and middle schools in 1994, Long Beach, Calif., officials said school violence plummeted, and attendance soared.

Opponents of uniforms point to studies that dispute such links. Lawsuits have been waged.

"Student dress should be considered some kind of protected expression," said Fritz Mulhauser, a staff attorney for the American Civil Liberties Union in the District. It should not be affected by "the inevitable generational gap of taste," he said.

At Northwestern High, a school with low student test scores and crowded classrooms, the uniform policy was put to a faculty vote and received 75 percent approval. Then 2,000 ballots were sent to students' homes; about 450 parents responded, with 84 percent in favor of uniforms.

Some parents who opposed the change aren't remaining silent. Patricia Cronauer, Isabelle Moreau's mother, said she plans to circulate a petition to get enough signatures to overturn the decision. Northwestern has serious academic problems, she said, but forcing students to dress a certain way won't change that.

Carl Barnes, who has two daughters at Northwestern, said: "I'm not for halter tops and all the other hoochie wear. I can't go for that at all. A dress code is fine, but a mandatory uniform? I'm not into that. I don't like people mandating what my children wear."

Acting Principal Jerome Thomas said the school staff will enforce the rules and offer such incentives as store discounts to students who wear uniforms consistently.

Diane Cabrera, 15, said she doesn't need such incentives because she's grown tired of peer pressure. "It was like a competition of who looked the best," she said as she filled out forms to transfer into Northwestern this month, wearing a T-shirt with the words "Queens," "glamorous" and "Bronx" emblazoned on it in pink, green and yellow.

Mario Macias, third from right, waits for a bus at 30th and Upshur streets in Mount Rainier with schoolmates from Northwestern, where khaki, navy or black pants, skirts or dresses and white, khaki, blue or black shirts are required. Mario prefers T-shirts and oversize jeans, but he's required to wear a polo-style shirt and khakis to Northwestern High School.