A heavily armed pupil injured his head teacher and two other people during a shooting at a French high school on Thursday, rattling nerves in a country repeatedly the target of Islamist militant attacks, police and officials said.

The 17-year-old pupil was arrested afterwards in possession of a rifle, two handguns and two grenades after the attack at the Tocqueville high school in the sleepy hillside town of Grasse in southern France, police told AFP.

The head of the regional government, Christian Estrosi, told AFP that the shooting was "not at all" being seen as a terror attack at this stage, adding that the principal and two other pupils were lightly injured.

There was conflicting information about whether a second suspect was on the run, with police initially saying they were looking for an accomplice. Another police source said the shooter acted alone.

"It was like being in a movie. We're not used to it, we hear about these things in Paris but not here. I was totally panicked," one pupil at the school, identified as Andreas, told BFM television.

France is still in a state of emergency after a series of Islamist militant attacks including the massacre in Paris in November 2015, claimed by the Islamic State (IS) group, and a truck attack in Nice, just 40 kilometres (25 miles) from Grasse, in July last year.

The shooting comes about 40 days before a two-stage presidential election in April and May in which security is one of the main issues on voters' minds.

The motive for the attack was still unknown.

Estrosi told France Info radio that the head teacher had been shot but apparently was not seriously injured.

"According to initial information that I have, we're not talking about an injury that could have life-threatening consequences," he said.

All schools in Grasse were locked down after the late-morning shooting, which led panicked students to flee the school and hide, local authorities said.

The French government has bolstered security outside schools following a series of Islamist militant attacks since January 2015 that have claimed hundreds of lives.

More than 3,000 reservists were called up to help keep watch outside the country's 64,000 primary and secondary schools for the return to the school year in September.

Prime Minister Bernard Cazeneuve cut short a trip to the northern Somme area because of the Grasse shooting, as well as a letter bomb blast at the offices of the International Monetary Fund in Paris on Thursday.

A secretary at the agency suffered burns to her hands and face after opening a parcel containing explosive material.

Employees were evacuated from the building near the Arc de Triomphe monument in the heart of the capital "as a precaution", a police source said.

IMF chief Christine Lagarde condemned it as a "cowardly act of violence".

US-style school shootings are almost unheard-of in France, a country with low levels of gun violence.

The last major attack in a school was in 2012, when an Islamic extremist from Toulouse, Mohamed Merah, shot dead three children and a teacher at a Jewish school in the city before being killed by police.

In March 1984, a 15-year-old student shot and killed a teacher in the southwestern town of Castres before turning the gun on himself.