Story highlights The teacher agreed to students' request to go on the slope, after a colleague rebuffed it

Romanian and Hungarian skiers may have inadvertently set off the avalanche, one skier fears

2 Lyon high school students and a Ukrainian skier died in the avalanche in the French Alps

Paris (CNN) A schoolteacher who went with students onto a closed French Alps ski slope before an avalanche struck there and killed three people -- two of whom were students he was in charge of -- faces charges of involuntary manslaughter, a prosecutor announced Thursday.

The physical education teacher accompanied 10 students from the St.-Exupery school in Lyon to a part of Les Deux Alpes ski resort that has been closed off since the start of the ski season -- at first due to lack of snow, then due to a high avalanche risk -- Grenoble prosecutor Jean-Yves Coquillat said.

Many signs and a safety net had been put up to keep people off the course. Yet the students and their teacher went over that net, said Coquillat, who noted that backcountry or off-piste skiing itself is not a crime.

The teacher, who is not a ski instructor, was among those caught up in Wednesday's avalanche, and was still in the hospital one day later with non-life-threatening injuries.

A 16-year-old girl was found dead on the closed ski slope, while a 14-year-old boy died after being transferred to a hospital. Another person outside the student group -- a Ukrainian skier, who appeared to be alone -- also died.

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