A 15-year-old boy lost his hand during a confrontation between police and demonstrators near a high school in Lens on Monday, as student protests over school conditions and staffing intensified across France.
Pas-de-Calais prefect François-Xavier Lauch said the teenager's life was not in danger and that he was quickly transferred to hospital.
An investigation has been opened into the circumstances of the injury.
The incident occurred near Auguste Béhal high school, where Lauch said a larger gathering than on previous days had assembled.
He said protesters threw stones, fireworks and mortars at police before riot officers were deployed.
French media have reported conflicting accounts of how the teenager was injured.
AFP, citing a police union source, reported that he appeared to have picked up a sting-ball grenade fired by riot police, while La Voix du Nord quoted a witness disputing that account.
Lens Mayor Sylvain Robert said the circumstances remained unclear.
A regional prosecutor said the General Inspection of the National Police, France's police watchdog, would investigate.
The school subsequently switched to distance learning and established a support unit.
Around 500 schools were partially or completely closed on Monday, while up to 40 university buildings were blockaded in support of the movement.
Education Minister Édouard Geffray said at least 190 students have been injured since the protests began.
Other cases have increased scrutiny of the police response.
A 16-year-old pupil in Tours lost sight in his right eye last week, with prosecutors saying his injuries were consistent with a police rubber bullet impact.
Separately, 14-year-old Eden suffered a double jaw fracture in Saint-Ouen-l'Aumône after reportedly being struck by a tear gas grenade.
Investigations are under way in both cases.
Prime Minister Sébastien Lecornu has acknowledged students' grievances as legitimate and ordered ministers to address demands involving absent teachers, lengthy school days and deteriorating buildings.
Proposals are expected by the end of the month.
Student unions have called for nationwide protests and school blockades on Tuesday, with teachers, trade unionists, public-sector workers and parents' groups expected to join marches across France.