The father who won a High Court battle over an unauthorised term-time holiday with his daughter wants to help other fined parents get their money back.
Last month, judges ruled in Jon Platt's favour after he refused to pay a £120 truancy penalty.
Mr Platt says hundreds of parents have contacted him since the case, believing they too were fined unlawfully.
The Local Government Association said the fines were issued in accordance with Department for Education guidance.
Isle of Wight Council fined Mr Platt £60 for taking his daughter on holiday to Florida without her school's permission in April 2015.
After he missed the payment deadline, the council doubled the fine to £120, which he also would not pay.
The council then prosecuted him for failing to ensure that his daughter attended school regularly, contrary to section 444(1) of the Education Act 1996.
But magistrates ruled he had no case to answer as, overall, his daughter had attended school regularly, with an attendance record of over 90% for the year - the threshold for persistent truancy defined by the Department for Education..
The council asked the High Court to clarify whether a seven-day absence amounted to a child failing to attend regularly.
At the High Court, Lord Justice Lloyd Jones and Mrs Justice Thirlwall dismissed the council's challenge, ruling that the magistrates had not "erred in law" when reaching their decision.
They decided the magistrates were entitled to take into account the "wider picture" of the child's attendance record outside of the dates she was absent during the holiday.
Now Mr Platt plans to help other parents pursue refunds.
He told BBC Radio 4's consumer programme, You & Yours, that he had set up a company to start group litigation.
"If local authorities across the country don't do the right thing, don't go through the history of these truancy penalty notices they have been issuing over the last few years and refund the money to parents where they issued it only in respect of a short unauthorised absence, then we will start a group litigation," he said.
"We will take tens of thousands of cases through the courts and local authorities will have to explain to a judge why they thought it was in their power to fine parents who had done nothing wrong."
The Local Government Association, which represents councils in England and Wales, said the fines were issued in accordance with Department for Education guidelines.
"The recent High Court ruling has created significant confusion amongst councils and parents and we are calling on the government to urgently provide clarity around the rules so that both parents and councils know where they stand," it said in a statement.
According to local authority data, almost 64,000 fines were imposed for unauthorised absences between September 2013 and August 2014.
Many parents say they are forced to consider term-time holidays as they are much cheaper.
But the Department for Education says there is clear evidence "that every extra day of school missed can affect a pupil's chance of gaining good GCSEs, which has a lasting effect on their life chances".
A spokesman said: "We are confident our policy to reduce school absence is clear and correct.
"We are examining the written judgment in detail to decide the way forward, but we are clear that children's attendance at school is non-negotiable and we will take the necessary steps to secure it."
Mr Platt, whose core business is in claims management, says his new company, School Fines Refunds Ltd is unlikely to make him any money, adding that its purpose is purely to seek refunds rather than compensation.