Baker Small, which tweeted gloating comments about families it had won cases against, has secured £3m in payments and contracts since 2010.
Critics said the spend was "depressing" and could have been used to help children rather than "fight parents".
Baker Small's Mark Small said he regretted the tweets posted in June.
But he defended the money paid to his firm for representing councils in tribunals to contest claims for funding and support by families of children with special educational needs (SEN).
A total of £3m was earmarked for - or paid to - the company by local authorities across the country between 2010 and 2016; £1.7m was earmarked or spent in the last year.
BBC analysis of Ministry of Justice data shows councils currently lose 86% of cases that go to tribunal, despite spending millions on legal advisors such as Baker Small.