She wrote on Facebook: "Niamh recently shaved all of her hair off to donate to a children's wig charity. I think this is the most courageous and amazing thing to do and makes me so proud.
"That's why I am so upset the school has made her feel so low and put her in to isolation because her hair needs to be 1cm longer.
"Niamh has always had outstanding reports and feedback from all of her teachers and everyone I know and she meets always says she is an amazingly polite and lovely girl.
"This doesn't change because of a hair style and to me it is discrimination. I'm actually fuming."
Defending the school's position academy principal Sara Davey said their policy on shaved haircuts was "very clear".
She has said Niamh must wear a head scarf until her hair grows back.
She said: "All students know this is the school policy and they also know that the consequence is to complete school work in the inclusion room until the hair grows so that it is no longer extreme.
"If Niamh had asked the school about shaving her hair for charity then this would have been pointed out and an alternative fundraising idea would have been suggested."
Ms Davey added: "The family had every opportunity to contact the school about their actions before Niamh shaved her hair for charity but they for some reason did not do this.
"Going forward, I will speak to Niamh's mother to try to resolve the situation to everyone's satisfaction so that Niamh can return to lessons as soon as possible.
"It would be reasonable in the circumstances to suggest that Niamh wears a head scarf until her hair grows sufficiently, for example."
MailOnline have contacted the Little Princess Trust for a comment.