In a first, France's highest heat warning was issued across half the country for Monday, with forecasters warning the scorching weather could end up being as serious as a 2003 heatwave that claimed the lives of nearly 15,000 people nationwide.Monday afternoon could see temperatures rise to 43C (109F) in the southwestern city of Bordeaux and 39C in Paris, according to weather service Meteo-France.More than 800 of the country's 60,000 schools were to be closed, while 1,800 others were to adjust teaching hours, the education ministry said.Since last week, other schools have suggested that parents keep their children at home or pick them up at lunchtime to extract them from boiling classrooms.A primary school teacher in the Bordeaux region, who asked to remain anonymous to speak freely, told AFP she was alarmed that the authorities were not taking measures to cool down schools."Last week, it was 32C in the classroom for the children."It's only going to get worse, while the supermarket across the street is cool and air-conditioned," she said."Everyone thinks it's normal but one day we're going to end up teaching in the aisles of the supermarket."Global warmingOn Monday morning, the Paris regional authorities urged residents to stay at home, warning disruptions were expected on public transport.