Quest Academy in Croydon, south London, said it was against its school uniform policy to allow head gear except for religious reasons.
So now Malachi Reid has no school to go to after his mum withdrew her 11-year-old son in protest.
The 'tam' hat is a round, tall hat, which is most commonly either knitted or crocheted. It is often brightly coloured and striped, for example with the colours of the Jamaican flag, and is used by Rastafarians to tuck their traditional dreadlocks away, as well as a way of identifying themselves as a member of the religion.
But Malachi, who had only just started at the school, was told his plain black hat - which he had worn without issue in his primary school - was against the rules.
His mum Beverley, 43, said the 'Tam' hat was a symbol of her son's religion, which is followed by more than one million people across the world and had told the school that her son wears his hat due to the family's Rastafarian beliefs.
But the school stuck to its guns and insisted Beverley wrote to them explaining its religious significance – a request she refused.
Principal Andy Crofts told the local newspaper: "We have a published uniform code which we apply consistently to everyone.
"We allow head coverings that are required for religious reasons. We have asked this parent to provide us with evidence that this headwear is a religious requirement on several occasions and she has refused to do so.
If we agreed to every parental request to modify some aspect of our uniform code, then it would cease to be a uniform.
But Beverley won't back down. She said: "I feel strongly about having to write in.
"I'm sure they haven't asked a Sikh to identify their religion or a Muslim to identify their religion.
"It's like a Freemason school and I'm not into sending my son to a place like that.
"Malachi's father works for the prison service, he works for the Queen, and he has got permission to wear his hat.
"This is my child they're talking about, they're trying to challenge his culture. It feels like we're in invisible chains."
She added: "At the moment, my son hasn't got a school. "It's like everyone's looking and thinking why is your little boy not at school? I could cry for him."
The Quest Academy hit the headlines in 2011 when it banned all physical contact between pupils including hugging and high-fives.
So who's more ridiculous - the school with its rules or the mother who's not prepared to write in to explain?