Izoduwa Montogue refused to allow her 4-year-old son to take part despite being told he couldn't be removed from the class at Heavers Farm School in Croydon.
She claims her family has been victimised since she made a complaint.
Mrs Montogue said: "After I complained about my young child being forced to take place in an event that goes against our Christian beliefs, the school's attitude towards me changed completely. I know other parents who are afraid to speak up because of how the school has treated me.
"It was like being bullied. They stopped treating me like any other parent but were antagonistic towards me. I believe that they retaliated against me by unreasonably excluding me from the premises, victimising my child and not taking my safeguarding concerns seriously.
"I wasn't even trying to stop the Pride event. I just wanted my child to receive an education, rather than indoctrination."
She's now taken formal legal action against the school and Croydon Council alleging a breach of her parental rights, victimisation and harassment at the hands of the school.
The parent has also filed a complaint with Damian Hinds, the education secretary, asking him to "use his authority to remedy the pervasive nature of LGBT proselytism within Heavers Farm Primary School".
Montogue is being supported by the Christian Legal Centre.
Spokesman Roger Kiska told Premier he's been shocked by her treatment.
He said: "She went into a meeting and the head's daughter who was taking notes was wearing a very inflammatory t-shirt, which said 'If you're homophobic or transphobic just be quiet'.
"Following that, two weeks later, the head teacher sent out the outcome letter and on that very day her son was put in detention for three hours.
"He'd never been in detention before. The following day, he gets another detention. So if that's not victimisation, I don't know what is.
"She brought her concerns forward. She was banned from the school yard and eventually pushed out of the school all together."
Premier has contacted Heavers Farm School and is waiting for a response.
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