When the school year starts next week, more than 11,000 New York City public school students will head to class in the city’s 18 brand new schools.

It marks the largest number of new schools and school seats to open in a single year under the city’s $13.1 billion, five-year capital plan. The schools are opening in all five boroughs.

Despite rising construction costs in the city, Mayor Bloomberg said yesterday that the average bids for new school construction now are “significantly lower” than before he overhauled the School Construction Authority after gaining control of the school system.

Over the past five years, the percentage of projects finished on time jumped to 80% from 60% and the percentage of projected completed within budget increased to 90% from 83%, he said.

“In fact, by every yardstick I think it’s fair to say that our capital plan represents a dramatic departure from the past,” he said yesterday during a press conference in the hallway of P.S. 307, a new school that is opening in Corona, Queens.

The cost of school construction has been a point of contention in the city and played a key role in the argument around giving Mr. Bloomberg control over the schools.

In 2002, as school construction costs soared above the cost of building similar-size luxury condominiums or specialized medical facilities, critics of the system argued that one of its main problems was that no single person was in charge.

Construction cost figures provided by the city, however, show that the cost of construction is growing, even though the mayor said the average bids for school construction are down compared with their level a few years ago.

Before Mr. Bloomberg took over, the cost of school construction was $400 a square foot. After changes to the School Construction Authority, the cost dropped to $325 a square foot, but that figure has now risen to $450 a square foot.

An aide to the mayor said yesterday that if the city hadn’t overhauled its design standards and adopted cost-effective engineering procedures, the cost of school construction would be $650 a square foot.

The city’s figure may be low, however. A recent report by the New York Building Congress and New York Building Foundation found that in 2007, it cost $512 a square foot to build a public elementary school in the city. By comparison, the cost of construction of a high-rise office building in the city came in at between $285 and $375 a square foot, the report found.

Building a new condominium tower in Manhattan costs less than a new school as well. A June study by the Manhattan Institute’s Center for Rethinking Development found that developers could expect to pay between $425 and $430 a square foot for a tower if construction were completed this year.

The New York Building Congress report noted, however, that as a general rule it costs more to build institutional facilities, such as schools and hospitals, because they “lack the repetitive template of residential and office structures, making development considerably more complex.”

The elementary school in Queens that Mr. Bloomberg visited yesterday, for instance, contains 37 classrooms, a 300-seat auditorium, a gymnasium, science labs, a multipurpose room, a cafeteria with a kitchen, music and art rooms, and a medical suite, as well as two playgrounds.

Mr. Bloomberg said yesterday that he plans to devote “100%” of his time — “if that’s what it takes” — to keep mayoral control intact. “I can’t think of anything more important. I can’t think of anything that would set the parents of schoolchildren in this city to man the barricades any more than stopping the progress that we have made,” he said.

It is scheduled to sunset in 2009, but the state Legislature has the ability to renew it.