A 12-year-old girl had her Christmas wish granted by a group of kindhearted high school students in Gallatin, Tennessee.

Her mother, Amy Howell, told her class at Gallatin High School that her daughter, who has a rare brain disease, loved school buses so much she asked for one for Christmas.

In less than a month, her 11th and 12th grade students raised enough money to transform an old school bus into a giant playhouse for Gabby.

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A 12-year-old girl had her Christmas wish granted by a group of kindhearted high school students in Gallatin, Tennessee

Her mother, Amy Howell, told her class at Gallatin High School that her daughter, who has a rare brain disease, loved school buses so much she asked for one for Christmas

'For me personally, it was just so sweet,' Amy Howell told ABC News.

'You put so much of your life into your students. To see it happen, that they want to give back like that, they don't want to do it for the glory, they want to do it because they love you. It was a really sweet thing.'

Jessi Smith, a 12th grader, said the group collected donations from the community and received help from a local middle school which provided art supplies and bean bag chairs to make the school bus a fun place for Gabby.

'Gabby, she ran up to me, she gave me a hug,' Jessi said.

In less than a month, her 11th and 12th grade students raised enough money to transform an old school bus into a giant playhouse for Gabby

Jessi Smith, a 12th grader, said the group collected donations from the community and received help from a local middle school

'She ran to Santa and said, 'I love you Santa, thank you so much.' She was screaming and she almost started crying and it was so sweet.'

Mrs Howell shared the story with her students and thought nothing of it.

'I told the story because I thought it was a funny thing for a kid to ask for,' she said.

Gabby was diagnosed with periventricular nodular heterotopia (PVNH) when she was just eight years old.

Gabby was diagnosed with periventricular nodular heterotopia (PVNH) when she was just eight years old

PVNH is a disorder that causes neurons in the brain to get 'stuck', preventing proper brain activity.

For Gabby, it means that although she grows physically, her brain is incapable of doing the same.

'Today, she can tell you the ABCs and tomorrow she can't. So she'll be like a little three-year-old for the rest of her life,' her mother said.

As well as a gift for Gabby, the group hoped the bus would serve as a token of gratitude to their teacher, Mrs Howell

Despite her disability, Gabby is popular among the Gallatin High School students and enjoys time spent playing with them when she stops by after school.

As well as a gift for Gabby, the group hoped the bus would serve as a token of gratitude to their teacher, Mrs Howell.

'She such a sweet person and she goes above and beyond every single day for all her students,' Jessi continued.