A Children's Court previously heard the teenager, 14, who cannot be named for legal reasons, threw about 15 rocks at the security guard breaking his nose and causing bleeding to the membrane between his skull and brain.
The court was told the teenager, and a co-accused, believed the security guard was a police officer and gloated about the assault shortly after.
“The security guard you can imagine what he’s going through, probably having nightmares and doesn’t want to do that job anymore,” a Magistrate told the teenager on Monday when sentencing.
“Sitting in the car and being pelted by rocks by people you don’t know - to sustain those injuries it's just a frightful thought don’t you think?”
The court was previously told the alleged incident occurred about 12.20 am on February 26 this year, following two thefts from the store in the days prior.
Police said the accused and co-accused attended the store on February 23 and stole $175 worth of alcohol.
The pair returned the following day and stole a further $236 worth of alcohol.
The court previously heard the teenager and co-accused were then involved in a spate of alleged vehicle thefts across Melbourne.
The teenager was denied bail following his arrest.
The teenager's lawyer told the court her client took part in a group conference following his arrest which she described as a "very emotional" and "beneficial" process.
The court was told the accused had written an apology letter which the lawyer said indicated his "remorse and his insight".
When sentencing, the Magistrate acknowledged the "serious set of circumstances" which brought the boy into custody.
“The consequences for the security guard could have been fatal or lifelong injuries,” the Magistrate said.
He acknowledged the teenager's participation in the group conferencing process, saying he was glad the accused took part.
“You need to make sure you don't let anyone down . . . be positive about your future and strong enough to say no and do positive things not negative things,” the Magistrate said.
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