A 13-year-old boy has been arrested in Spain for allegedly shooting a teacher dead with a crossbow at his school.

The unnamed pupil opened fire with the weapon at the Joan Fuster school in Barcelona at around 9.20am local time and unconfirmed reports claimed he was also armed with a machete, BB gun and material to make Molotov cocktails.

Two students and two teachers were wounded and the victims were taken to hospital while the school was shut down and traumatised children were taken away for counselling.

Witnesses claimed the boy knocked on a classroom door and immediately attacked a languages teacher with the crossbow as it opened, before stabbing her 13-year-old daughter.

The student reportedly told the screaming pupils in the class to "calm down", adding "I am not interested in you", La Vanguardia reported.

Hearing the uproar, another teacher ran into the room to help his colleague and was shot in the abdomen, El Pais reported. He was pronounced dead at the scene.

The man was said to be working as a substitute social sciences teacher at the school in the Sant Andreu district of Barcelona.

One student said the attacker targeted one of her friends but they managed to escape.

 "The boy came into our class with a knife and wanted to attack a friend of mine. But he didn't manage to, he left, and we had a chance to run away," the pupil told Reuters TV.

Children barricaded themselves into nearby classrooms and reportedly only evacuated when a fire alarm was set off, unaware of what had occurred.

"We were just starting the class and suddenly we heard screams," said student Gemma Jarque. "So we shut ourselves inside our classroom in order to be safe.

"We saw the teacher, lying on the floor in a pool of blood."

Another pupil told El Mundo newspaper that the boy had previously threatened to murder teachers before killing himself.

Classmates claimed there was a collection of weapons at his home and the teenager aspired to join the military, telling La Vanguardia that he wrote "blacklists" of pupils and teachers he disliked.

"He didn't have problems but he was kind of a loner," said Paula Amayuelas, who knows the suspect. "Other students would pick on him."