The city is opening a full-fledged high school for gay, lesbian, bisexual and transgender students -- the first of its kind in the nation, The Post has learned.

Operating for two decades as a small alternative program with just two classrooms, the new Harvey Milk High School officially opens as a stand-alone public school with 100 students in September.

The school is undergoing a $3.2 million in city-funded renovations approved by the old Board of Education in June of last year. It will eventually take in 170 students by September 2004, more than tripling last year's enrollment.

The Hetrick-Martin Institute -- the gay-rights youth-advocacy group that manages and helps finance the school in conjunction with New York's Department of Education -- has hired the school's first principal.

In the past, Harvey Milk High School  -- named after the slain gay San Francisco politician -- was assigned an "off-site" supervisor who also oversaw several other schools.

Principal William Salzman said Harvey Milk will be an academically rigorous school that will specialize in computer technology, arts and a culinary program.

"This school will be a model for the country and possibly the world," Salzman said.

"This is a not a touchy-feely situation. We intend to have 95 percent of our students go on to college. We have a lot of talent coming into the school. We want to steer these kids in the right direction."

But New York State Conservative Party Chairman Mike Long blasted the school as "social engineering" that wastes tax dollars.

"Is there a different way to teach homosexuals? Is there gay math? This is wrong. This makes absolutely no sense," Long said. "There's no reason these children should be treated separately."

Long said there are city and state discrimination laws on the books and that authorities should enforce them to stop gay-bashing.

"What next? Maybe we should have schools for chubby kids who get picked on. Maybe all kids who wear glasses should have special schools. It's ridiculous," he said.

What burns Long most is the $3 million spent on renovations. "Maybe this is one of the reasons the city has no money," he said.

Arthur Larsen, who graduated from the program last month as valedictorian, is thrilled with its expansion into a full-fledged school. "I'm now an alumnus of a real school!" he said. "There's going to be more students. In four years, I want to work here."