Mayor Michael Bloomberg welcomed students back to school Thursday and announced a new text-messaging system to help parents keep on top of their children's education.

"We're helping our kids succeed in giving their parents more tools and support," Bloomberg said, "and that includes using technology to help keep parents informed even when they're on the go."

Schools Chancellor Dennis Walcott, who has a grandson in a Queens elementary school, demonstrated how to sign up for texts about upcoming school events by texting "nycschools" to 877-877.

"Nycschools, right?" Walcott asked. "And push send. I'm signed up, and as a grandparent I will get information."

On the first day of classes for most of the city's 1.1 million public school students, Bloomberg joined Walcott at a newly opened Bronx complex that houses a new elementary school, a middle school that will expand to 12th grade and a special-needs school.

"Our schools are heading in the right direction and this year we're going to have the opportunity to do even better," Bloomberg said.

Visiting a ninth-grade math class, he told students he always liked math "'cause there's a right and a wrong answer."

Walcott went on to visit a school in each of the city's other four boroughs.

One challenge city schools face this year is making sure the 860,000 school lunches they serve daily deliver good nutrition while meeting federal guidelines. The city's efforts to fight obesity by slimming down school lunches has come in conflict with U.S. Department of Agriculture guidelines that mandate calorie minimums.

Asked about the issue, Bloomberg touted his administration's efforts to reduce childhood obesity.

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"I'm happy to say the young people in this city in the last three years, obesity has gone down something like 5 percent," Bloomberg said.

Bloomberg's education agenda suffered a defeat over the summer when an arbitrator blocked his plan to close 24 low-performing schools, replace half their teachers and reopen them with new names.

Asked about the ruling, Bloomberg called it "a disgrace."

"We are making it infinitely more difficult for those kids to get the kind of education they're going to need to join and share in the great American dream," he said.