Ellie Sandywell, 11, had the band confiscated by teachers after she insisted on wearing it in lessons.
She says she wears the bracelet because her 17-year-old cousin, Ethan Millard, is serving in the RAF, and her brother, Joshua Brayford, aged 20, is in training for the Royal Marines.
Sara Stevenson, executive headteacher at the University Primary Academy in Kidsgrove, Staffs, claims the wristband could endanger pupils.
She said: "While the Academy fully supports many charities, we also have to maintain high standards in health and safety and in presentation.
"Rubber and plastic wristbands are not permitted in school because, not only do they not form part of our school uniform, but if they were to get caught on items such as door handles they may cause injury."
Pupils are allowed to attach their wristbands or other decorations to school bags to show their support for their chosen charities."
Ellie said that despite the rule, she is determined to continue wearing the wristband.
She said: "I was given a choice of handing over my band or getting a red letter, which is something you get for bad behaviour.
"I was told off in front of all the class and I just wish I could have been quietly spoken to about it and I could have explained that I wear it in support of my cousin and brother.
"I am still going to wear it, but I will have to keep my blazer on so the teacher can't see it."
Ellie's mum, Elizabeth Hassall, said: "She rang me after school on the day it happened and she was in tears. I was worried about what had happened, and then she told me that her wristband had been confiscated by the school. I couldn't believe it.
"I know that things like earrings and nail varnish are not part of the uniform, but I think a charity wristband is a bit different.
"I went into school and spoke to the teacher about it, and was told they would continue to confiscate it because she's breaking the rules."
The 41-year-old added: "Ellie is a good girl and she always abides by the regulations when it comes to her uniform, but she feels very strongly about this cause.
"In my eyes, wearing a wristband is exactly the same as wearing a poppy, so what are they going to do when it comes to Remembrance Day?
"I can't remember a day where Ellie hasn't worn her wristband and I think it's quite nice that she's so young and understands the importance of wearing it all the time."