The National Union of Teachers spent more on campaigning in the run-up to last year's general election than either UKIP or the Green Party.
It spent £326,306 in the year up to the 8 June 2017 poll, compared with UKIP's £273,104 and the Greens' £285,662.
It said its campaign opposing school funding cuts had "delivered results" by forcing ministers to find more money.
The Conservatives outspent Labour by £18.5m to £11m during the recorded period while the Lib Dems spent £6.78m.
The 2017 election resulted in a Hung Parliament and the Conservatives having to ditch a number of manifesto proposals including scrapping free school lunches for infant children from better-off families.
The NUT, which has since merged with the ATL to form the National Education Union, said its spending had been justified in forcing changes to education priorities.
"Our election spending was on the successful school cuts campaign which made school funding a high profile election issue, changed 750,000 votes at the election, and resulted in the government stumping up another £1.3bn for schools in July," Kevin Courtney, general secretary of the NUT section of the NEU, said.
The Electoral Commission figures outline recorded spending of more than £250,000 by political parties and non-party campaigners during the regulated period, between 9 June 2016 and 8 June 2017.
They show that the anti-Brexit group Best for Britain, founded by campaigner Gina Miller, spent £353,118.
Both the NUT and Best for Britain are under investigation by the watchdog for submitting an incomplete spending return while Best for Britain is also facing questions for not returning a £25,000 donation from an impermissible donor within the 30 days required by electoral law.
Who spent what during the 2017 general election period
- Conservative and Unionist Party: £18,565,102
- Labour Party: £11,003,980
- Liberal Democrats: £6,788,316
- Scottish National Party: £1,623,127
- Best for Britain: £353,118
- National Union of Teachers: £326,306
- Green Party of England and Wales: £299,352
- Women's Equality Party: £285,662
- UKIP: £273,104
The watchdog is also investigating potential breaches by the Conservatives, Labour, and Green Party relating to statements of payments made and by the Conservatives and Lib Dems for late payments to suppliers.
The Women's Equality Party - which spent £285,662 - is under investigation for registering a spending return that was inconsistent with its reported donations.
In all these cases, the Electoral Commission said it would determine whether electoral law had been broken and impose sanctions if necessary.
But the watchdog said the major political parties "may wish to consider the robustness of their internal governance and level of resourcing to ensure they can deliver what the law requires".
Prime Minister Theresa May's decision in April last year to call a snap election caught many people unawares.
Nevertheless, total spending in 2017 - including by parties and campaigners which spent less than £250,000 - totalled £41,587,450, more than £2m higher than at the 2015 election.
Labour, UKIP and the Green Party all spent less than during the 2015 campaign but the Conservatives' expenditure increased by 19% while the Lib Dems spent 83% more than two years earlier.
The SNP spent £1.62m during the recorded period, about 10% more than during the 2015 poll.