THE word traditional is not often used when describing New York City's Manhattan International High School. The desks in the dimly lit classrooms are not arranged in the grid pattern of conventional schools, but in ovals so four students can work together on projects. The teachers do not lecture about history or math for 40 minutes but lead discussions about Revolutions and Encounters for 100 minutes. And the international composition of the student body would make even UN Secretary-General Boutros Boutros-Ghali sit up and take notice. Olga Marte and Ramon Castillo are from the Dominican Republic, Marcin Tomasik and Nina Sosnicka are natives of Poland, and Alan Sadovnikovas is of Lithuanian descent. More than 40 countries are represented at the school, which admits only students who have been in the United States less than four years. But what makes this school stand out from the many other multicultural public institutions is its innovative scheduling format. Manhattan International is one of the first schools in the country to successfully bring block scheduling - the use of two or more periods for the extended exploration of complex topics - into an urban public school system. Between 10 to 15 school districts in New York State currently administer some form of block scheduling, estimates Alan Ray, director of communications for the New York State Education Department, and there are positive signs that it spurs more in-depth, college-like discussions in the classroom. ''We get to more, get more depth,'' says Olga, an 11th grade student at Manhattan International. ''This school is different from other schools because we learn and share ideas with each other as well as with the teachers. With the old schedule, by the end of the day, you had to try to remember what homework you had for first period.'' BLOCK scheduling is gaining support not only in New York City but also around the country, because it is a low-cost, long-term way of improving America's financially strapped public school system. ''Schools have to make a commitment to systemic, long-term change, and block scheduling is a significant way of improving their overall standards,'' says Charles Santelli, director of policy for the New York State United Teachers, a union. In a block-scheduling study done at Brockport High School in upstate New York over the 1993-'94 school year, Donald Nelson-Nasca, a recently retired professor of educational administration who has studied block scheduling, found that the school gained 42 days of instructional time over the course of a school year by implementing the concept. ''For every hour of the school day, almost 10 minutes of potential instruction time is wasted when students pack up their bags and head for the next class,'' says Dr. Nasca. The longer, less chaotic schedule gives teachers the opportunity to work one-on-one with their students. ''Rather than being thrown 125 kids who race in and out all day,'' Brockport English teacher Jack Casement recently told New York Teacher magazine, ''I have the chance to really get to know them.'' The Brockport study also found that block scheduling helped reduce discipline problems (by 13 percent), increase the passing percentage for state Regents exams (by 10 percent), and drastically reduce the number of days teachers missed (by 22 percent). Manhattan International's teachers, who develop their own curriculums because students are at such different levels of English proficiency, do not even have time to think about taking a day off. ''It takes a lot of extra work on our part to teach with block scheduling, but everybody who works here chooses to because it's worth it,'' says Bridgit Bye, Manhattan International's internship program director and one of the founders of the school's curriculum. ''We obviously haven't been successful using the traditional way of learning. I would never go back to a traditional school again.'' At Manhattan International, adding one hour to a cluster class called Revolutions - which incorporates art, science, and the humanities - affords students the time to learn the meaning of certain words and the chance to complete ambitious projects. ''The extra time gave the students who know less English a chance to ask language questions, and it also allowed us to work in small groups on our projects,'' says Mugeeb Algaithi, an 11th grader who was born in Yemen. ''In 40 minutes you can't get nothing done.'' Last year, student projects included assembling books on religion (Confucianism), discovery (Christopher Columbus), and evolution. While block scheduling has shown promise, Mr. Santelli warns it may not be as successful at large urban public schools, because they often lack the teachers to engage students for that long a period. ''Given the fiscal crises plaguing our cities, changes of this type are just like pushing a large rock up a hill,'' he says. ''It is almost impossible to put in these changes when supplies are in short supply and class sizes are so much bigger in the city than in the rest of the state.'' Block scheduling is only in its third year at Manhattan International, making it too early to tell how well the format works over the long haul. So far, however, Bye says, ''In all my years of teaching, I have never seen the beginning students' scores jump like they have here.'' * Previous articles in this series on urban education appeared on Sept. 7, 12, 14, 19.