Scoured and disinfected, most of Mexico's primary schools and kindergartens stood ready to welcome back millions of students Monday after a nationwide shutdown ordered to help put a brake on the spread of swine flu. Children who turned up with symptoms of the illness would be turned away, education officials said. But it wasn't the countrywide restart that leaders had hoped for. Six of Mexico's 31 states put off reopening schools for another week amid a rise in suspected flu cases in some regions, and a seventh ordered a one-day delay. Some parents were worried about sending their children back so soon. While Mexicans are feeling a little more relaxed, the swine flu outbreak is continuing to spread around the globe, with international health authorities reporting more than 4,500 confirmed cases in 29 nations. There are 53 deaths tied to the virus - 48 in Mexico, three in the U.S., one in Canada and one in Costa Rica. The United States now has the most confirmed cases - 2,532 in 44 states, more than 900 ahead of Mexico's total, the US Centers for Disease Control and Prevention said Sunday. In Mexico, crews worked through the weekend to cleanse school buildings and make sure they were stocked with sanitary supplies as 25 million children prepared to resume their studies after a two-week break that began when authorities ordered schools closed in the Mexico City region on April 24 and then the whole country three days later. "We have cleaned the windows, classrooms, blackboards, floors, bathrooms, everything," Flor Carpio, whose husband is the custodian at Mexico City's Horacio Mann grade school, said Sunday. At the Rosaura Zapati day care center in central Mexico City, Miguel Sanchez cleaned a staircase with bleach even though he was not totally convinced that swine flu existed. "To me, it only exists in the mind of (President Felipe) Calderon and the WHO. It is a vile lie," said Sanchez, the center's caretaker. The federal Education Department said Sunday that 88.9 percent of the nation's estimated 250,000 schools had been cleaned and disinfected. A day earlier, Secretary of Public Education Alonso Lujambio urged parents not to send their children back to school if they were sick and told teachers to be on guard for possible swine flu cases. "School life will return to normal as long as the safeguards we have put in place are effective. Help us in this," Lujambio said. His department said Sunday that groups of teachers and parents would be waiting at entrances to identify any students who showed up with flu symptoms. Any who did would be sent back home, but "without stigmatizing the children or violating their rights," it said. Mexican health officials say swine flu has been confirmed in 1,626 people, of whom 48 have died. Suspected cases were reported after those numbers were released Saturday, but the government offered no new count on confirmed cases Sunday. Because of the new suspected cases, the states of Jalisco, Hidalgo, Guerrero, San Luis Potosi, Chiapas and Zacatecas postponed the resumption of classes until May 18. Michoacan said its schools would reopen Tuesday. High schools and universities restarted last Thursday. The reopening of kindergartens and primary schools is the latest step in Mexico's efforts to restore a sense of normality after the flu scare. Businesses, restaurants and bars gradually resumed operations over the past week, and except for public servants and restaurant workers, it is less and less common to see people wearing surgical masks. While Mexico's schools were getting their last-minute sprucing up Sunday, the government ratcheted up its ill feelings toward China over the treatment of Mexican citizens because of the swine flu outbreak Officials said the country would not participate in a Shanghai trade fair May 19-21 as planned because China had withdrawn Mexico's "guest of honor" status. Thirty Mexican companies had been scheduled to take part. Mexican officials were already angry over China's quarantining of dozens of Mexican travelers, airline flight cancellations and a ban on its pork products - moves that were part of a wider series of snubs by many nations that has left Mexico feeling unfairly singled out. "The recently adopted measures by fair organizers and the Chinese government are unacceptable," ProMexico, the government's agency that promotes foreign trade, said in a statement. China has defended the steps as necessary to keep swine flu out of the world's most populous nation. But on Monday, China reported its first confirmed case of swine flu on the mainland - a 30-year-old Chinese student who had returned from studying at a college in the United States. The Chinese government quarantined dozens of other people who were on a flight with him. Mexico said Sunday that 13 Mexicans remained in quarantine in China and one in Singapore. Last week Mexico chartered a flight to bring home dozens of its citizens from China. It was unclear if the 14 mentioned Sunday had been placed under restrictions in China since the first group was brought home.