Police have charged a 15-year-old boy after a video allegedly showed a student with autism being attacked by a group of spanner-wielding teenagers outside a Melbourne school.

The 14-year-old, who cannot be named for legal reasons, was allegedly attacked outside a school in the city's inner-north, on Tuesday afternoon.

His alleged assailants had armed themselves with spanners and are accused of turning on another boy when he tried to help the victim.

Police today charged a boy from Melbourne's north with affray, recklessly causing injury and intentionally causing injury.

The teen has been bailed to appear at a Children's Court.

Earlier, the boy's mother said teenagers had targeted her son after he stood up for a friend who was being bullied.

The Education Department said violence at schools was not common.

"Incidents of violence and aggressive behaviour remain relatively rare in Victoria's more than 1,500 government schools," a statement said.