A teenager accused of bashing an autistic boy alongside two spanner-wielding friends was "antagonised" on Instagram before the fight, a Melbourne court has been told.

The 15-year-old, who is on bail, is likely to avoid a criminal conviction if he is deemed suitable for a diversion program to take his matter out of the court system.

The teen, who cannot be named for legal reasons, appeared in a children's court on Tuesday supported by his mother and sister.

It is claimed he was messaged on Instagram by his 14-year-old victim before the alleged incident outside Northcote High School on May 8.

The victim wanted to fight over an alleged bullying matter regarding a friend, police said.