By the time you read this, hundreds of city students will already be back in their classrooms.  At least 19 of the 60 city charter schools start classes in August.One, the South Bronx Classical Charter School, tacks two weeks onto the beginning of the academic year – as well as taking a shorter spring break.

“A lot of it is a function of getting the students in early so that we can be more prepared for city and state tests,” said South Bronx Classical’s Executive Director Lester Long of the choice to begin early instead of ending late.

An August start creates obstacles for administrators, though, including adding to the cost of busing. Charter schools can use city school buses, but they don’t generally start until after Labor Day. Charter schools pick up the tab for the added busing.

“Busing is expensive,” said Principal Charlene Reid of the Bronx Charter School for Excellence, explaining why her school starts only a couple days before public schools.

Charter schools have more classroom time than public schools, said charter-school advocates, by having longer school days, longer school years and summer or vacation sessions.

At least one charter school operates 200 days a year – compared to the city public schools’ maximum of 185.  “That’s been a staple,” said Jeff Maclin, spokesman for the New York City Center for Charter School Excellence.

When Gov. Spitzer outlined his plan for education reform this year, he included a call for a longer school year or school day as a way to improve schools.  “He’s taking concepts that are very typical of charter schools and adopting them,” said Peter Murphy, policy director of the New York Charter School Association.

Union contracts are often considered to be an impediment to lengthening the school year.  If the Education Department wants to add more sessions to the state-required minimum of 180 school days, it must negotiate with the teachers union.  The 2005 union contract sent students back to class the day after Labor Day for the first time in memory. In some grades, public school students attend 185 days of classes.

United Federation of Teachers President Randi Weingarten said it wasn’t the union that objected to a longer school year.  “Parents and camp owners fought tooth and nail against it,” she said, adding that summer should be used for enrichment, recreation and summer school.  The UFT runs two Brooklyn charter schools which open next Tuesday – the same day as city public schools.