The boy opened fire at Northeastern College in the city of Monterrey, civil protection official Oscar Aboytes told AFP.

He said the school initially told the civil protection service that three people had been killed, but when experts arrived at the school, they found those victims to be "extremely seriously" injured.

Mexico suffers regular gang violence but so far nothing appeared to indicate that the school shooting was gang-related.

"It is an unprecedented situation. Nothing like this has happened before" in schools in the surrounding state of Nuevo Leon, state security secretary Aldo Fasci told reporters.

Wednesday's attack came as the northeastern state of Quintana Roo, hundreds of miles from Monterrey, reeled from two shootings, thought to be linked to gangs or drug dealers, that left nine people dead.

In 2014, 43 students from a teacher training college in southern Mexico went missing and are believed dead.