Claire Wright says she went into shock after receiving the call from a First Aider at McAuley Catholic High School while she was at work on Monday.
She later learned that her son, Aaron Matthews, aged 12, had been hit on the back of the leg with a ball bearing that had been fired from a homemade weapon that was made using a plastic bottle and a balloon.
"Finding out in that way was awful. It was so insensitive for someone to come on the phone and just say that to me. It was only after they put Aaron on the phone to me and he explained what had happened that I realised the ball bearing hadn't gone through the skin. But I didn't know what to think when they phoned and said that at first."
"I work with vulnerable older people, through my business, Claire's companions, and was in the middle of work when I got that call," said Claire of Kingsley Crescent, Armthorpe.
Claire kept Aaron off school on Tuesday due to concerns about his welfare, and says she wants disciplinary action to be taken against the pupil who fired the makeshift weapon - who is also believed to have hit a number of other pupils with it.
"I think the boy who did it should be excluded for a couple of weeks, myself. He's made a weapon and brought it into school, with the obvious intention of hurting others," explained the mum-of-three.
The 44-year-old added: "The school say they think there were a number of other children involved, and that they are taking it seriously."
A spokesman for McAuley said: "A pupil was investigated for foolish behaviour with ball bearings, but there was no gun."
This comes after members of the public contacted the Free Press with reports of pupils at the school being shot with a BB gun.
Further information on the incident, or the pupil investigated has not been provided by the school, but the spokesman said a gun was not used and no-one was injured.
The spokesman said no further action was being taken against the pupil involved, and confirmed that parents were not informed of the 'minor' incident because it was not serious enough to warrant that kind of response from the school.
A South Yorkshire Police spokesman said the force was not contacted in connection with the incident.