The victim was a woman teacher of mathematics and science at the Acheuléen “lycée professionel” near the train station of the northern French city.
The incident occurred around 2.30pm as the woman was teaching a class, according to press reports.
The teacher, opening the classroom door after someone knocked, was sprayed with a teargas grenade by a young person wearing a hood, whose face was partly hidden, the Courrier-Picard newspaper reported, citing another teacher.
The attacker fled, leaving the teacher in a state of shock.
She was transported to the emergency department of the local hospital.
It is not known whether the assailant was enrolled at the school of 750 students.
The entire staff of the school decided to discontinue classes later in the afternoon over concerns about the lack of security at the institution.
They were scheduled to meet on Wednesday to decide what further action to take.
The head of the school, who declined comment on the incident, has lodged a complaint with the national police, the Courrier-Picard newspaper said.
Jean-Louis Ducrocq, an elected representative of the teachers, said it was not the first time that the school had faced such violence and more needed to be done to control access to classrooms.
Claude Legrand, a local school inspector, decried what he called a “reprehensible and scandalous act”.