Deja Taylor, 26, pleaded guilty in August to child neglect, as well as to federal charges of possessing a firearm while using cannabis and making a false statement while purchasing a pistol in June.

After being sentenced to 21 months for the other two charges in November, the mother from Virginia will now spend a further two years in prison for felony neglect.

The combination of the state and federal sentences means the mother will spend nearly four years behind bars.

Taylor's son, who has not been identified, used a 9mm semi-automatic pistol to shoot Abigail Zwerner, a 25-year-old first-grade teacher, in January.

Police said the boy had taken the handgun from home, placed it in his backpack and removed it while Ms Zwerner was teaching her class at Richneck Elementary School in Newport News, Virginia.

The child then fired one shot, which pierced Ms Zwerner's hand and entered her chest.

No one else was injured and the teacher was hailed a "hero" for evacuating students to safety after being shot.

The teacher underwent multiple operations in around two weeks.

She has since undergone five hand operations and still has the bullet in her chest, according to The Virginian-Pilot newspaper.

Sky News partner network NBC News reported in January that Ms Zwerner texted a loved one an hour before she was shot to say the boy had a gun.

A source close to Ms Zwerner told the broadcaster that the teacher felt administrators at Richneck Elementary School in Newport News weren't helping with the situation.

Read more:
Zwerner said recovery is 'exhausting'

Taylor: Guilty 'for the rest of my life'

In May, Taylor told ABC's Good Morning she feels responsible for the shooting and apologised to Ms Zwerner.

She said at the time: "That is my son, so I am, as a parent, obviously willing to take responsibility for him because he can't take responsibility for himself."

During her sentencing in federal court in November, one of Taylor's lawyers read aloud a statement in which she said she would feel remorse "for the rest of my life."

Her defence pleaded for leniency and a maximum of six months in prison, and also said that Taylor had trouble with addiction and mental health issues which "will only be exacerbated by imprisonment".

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House explodes as police surround armed suspect inside

Zwerner suing school for $40m

Ms Zwerner meanwhile filed a $40m (£32m) lawsuit against Richneck Elementary in April, and accused management of ignoring warnings about the boy's behaviour.

"Teachers' concerns with John Doe's behaviour was regularly brought to the attention of Richneck Elementary School administration, and the concerns were always dismissed," the legal filing says, using the name John Doe to avoid naming the child.

"Often when he was taken to the school office to address his behaviour, he would return to the classroom shortly thereafter with some type of reward, such as a piece of candy."

USA Today reported in November that Newport News circuit court judge Matthew Hoffman ruled against the school's motion to dismiss the legal action.