The school's principal said the teacher remained in "very worrying" condition after her suicide attempt on Thursday, as teachers' unions said her desperate act highlighted their difficult working conditions.
The 44-year-old teacher, whom French media have identified only as Lise B., doused herself in petrol during the morning break and set herself alight, a day after a stormy meeting with children in her class who found her teaching methods too strict, witnesses said.
Christian Philip, the principal at Jean Moulin secondary school in Beziers, southern France, said the school had reopened its doors on Friday morning but that classes remained suspended until at least Monday.
Traumatised students were being assisted by a psychological crisis cell, he said, and about 80 students had already been treated. The cell was to stay in place until at least Tuesday.
Philip said the state of the teacher's health was "still very worrying" but he had no further details on her condition.
Local prosecutor Patrick Mathe meanwhile said an initial investigation had found "no criminal act" connected with the incident.
In a joint statement, unions representing secondary school teachers said the incident underscored the hardships facing teachers and called for a public debate on working conditions.
Noting the "significance of the choice of the workplace to commit this desperate act," the unions called on Education Minister Luc Chatel to organise public consultations on "the realities of (teachers') work."
"We must be aware of what is being called teachers' fatigue, of professional problems, of the suffering at work that, while we see it in other professions, is more and more present within the education system," the unions said.
The teacher suffered third-degree burns and was taken from the school -- a huge institution housing 3,000 students and 280 teachers -- by helicopter to a specialist unit in the city of Montpellier's university teaching hospital.
Parents and pupils who spoke to AFP at the scene Thursday said the teacher had a difficult relationship with several pupils in her maths class and that a meeting with them to clear the air on Wednesday had become rowdy.