WHEN Los Angeles school district planners began drafting a proposal for their first new high school in 20 years, they abandoned the prosaic six-period days and multiple-choice tests. They chose, instead, to stretch the envelope of education, dreaming up a new kind of school that would teach 5,300 students in eight ''career academies'' and involve local business to a degree rarely seen in public high schools. Scheduled to open by 1999, the high school is touted by district officials as a model for the future. It will emphasize career planning, advanced partnerships with businesses, and pioneer a new method of financing the school's $65 million cost. ''In every case, districts build schools and put programs in them,'' says Dominic Shambra, the district's development director. ''This school is designed to reflect its curriculum, and that's what makes it different.'' At the school, students will choose a ''major'' such as law, communications, or engineering. Their math, English, and science classes will be taught through the lens of this specialty. If a student were in the law and government career academy, for instance, the teaching of Harper Lee's ''To Kill a Mockingbird'' could come alive, revealing the inner workings of the courtroom as well as the lessons of literature. Students can also expect internships with prestigious law firms, summer jobs at neighboring hotels, or hands-on experience at day-care centers and health clinics housed right on campus. Magnet schools across the country have long focused their curricula on certain subject areas. But the proximity of this school to Los Angeles's bustling financial district, the plan to include small businesses on campus, and the task force of community members and educators involved in the school's design and curriculum from the beginning make the Belmont Learning Complex proposal one-of-a-kind. ''The community is very excited about all the different components,'' say Linda Del Cueto, the district's administrative coordinator for planning and development. ''The question I always get is, 'When will this open?' Then I see mental calculation as a parent thinks, 'How old will my child be then?''' This dream may be about to hit a wall of reality, however. In early September, the school board is scheduled to vote on whether the district may select a developer and begin negotiations. Most contentious could be teacher unions, which are vowing to fight the district's preferred developer, saying that the Japanese firm is under investigation in its home country for fraud and using slave labor. The school's size is also drawing criticism. The career academies, together with a middle school, a program for immigrant teens, and an existing magnet school could boost the student population to nearly 9,000, raising security concerns. Education experts also caution against the number of students planned for the school. ''There's a great deal of research today that indicates that large schools - no matter what their focus - don't work,'' says Tony Wagner, president of the Boston-based Institute for Responsive Education. ''This has the potential of being a white elephant, a boondoggle.'' But according to Michael Kirsch, an education professor at Stanford University, career academies throughout California have reaped impressive results. Career academies are successful because of the opportunities and camaraderie they provide for a cluster of students, Mr. Kirsch says. ''They provide on-the-job training and up-to-date equipment for schools, which, in Los Angeles, is really hard to come by.'' But even he has concerns about the number of students. District officials dismiss their critics by saying their plan allows them to break the huge school into manageable units ''How do you make small out of large at [the University of California at Los Angeles]?'' asks Mr. Shambra. ''They have majors. They deal with the students on a smaller level.'' The school will offer majors in areas such as international studies, communications, humanities, law, tourism, health professions, engineering, and business. This kind of program will allow students - largely from Hispanic, lower-income homes - a leg up in life, planners say. ''We need to prepare these kids for the future,'' Shambra says. ''The public and business are always quick to criticize that students are lacking in skills. Now, we're saying, 'Let's get together and build these skills into the instructional program.'''