An aerial shot of emergency service workers in uniforms and helmets standing on a platform beside two train carriages.

Emergency services attend to the boy who was trapped under a train at North Melbourne Railway Station. (ABC News)

A school has told parents that an incident at North Melbourne Railway Station yesterday, which left a student seriously hurt, was caused by the boy’s bag getting caught on the passing train’s mirror.

St Aloysius College has provided an update after a year 7 boy was trapped under a train carriage for up to an hour on Monday afternoon, suffering serious leg injuries.

A paramedic who responded to the emergency incident described the scene as “traumatic”.

The college said it had received information describing how the incident unfolded.

“The student was standing with friends near the edge of platform 6 as a train arrived, when their school bag became caught on the train, causing the student to fall onto the track,” the school’s principal, Mary Farah, said in the email to parents.

She said a number of other students witnessed the incident and immediately sought help for the boy.

“A fellow student pressed the platform emergency button and contacted Triple Zero (000), while other students communicated clearly with transport authorities.

“The college commends the calm and responsible actions of the students involved.”

People stand behind entry gates to a railway station under a sign that says "North Melbourne Station" in the late afternoon sun.

The incident happened as school students were catching trains home from North Melbourne Railway Station just before 4pm. (ABC News)

Ms Farah said senior members of the school’s leadership team rushed to the station to support the students who witnessed the incident.

“The college wellbeing and pastoral care team is providing counselling and support to students, staff and families,” she said.

“St Aloysius College extends its sincere gratitude to first responders and emergency services personnel who attended the scene, and acknowledges the calm, responsible and decisive actions taken by students present during this distressing situation.”

Metro Trains said an investigation was underway into the incident.

“This is a tragic accident involving a young student, and our thoughts are with his family, friends and school community,” a Metro spokesperson said in a statement.

“We thank emergency services and staff for their quick response.”

A Fire Rescue Victoria spokesperson yesterday told the ABC the boy was trapped by his leg until firefighters were able to jack the train off the tracks.

The boy was rushed to The Royal Children’s Hospital in a critical condition, but this morning the hospital advised his condition had improved to “serious but stable”.