Kirsty Williams said more money was being spent per pupil in England which was an "inequality" in the system.
She was speaking before addressing the Liberal Democrats' UK conference in Glasgow on Monday.
The Welsh government said it was providing funding to support disadvantaged pupils.
Ms Williams said education would be a key Lib Dem priority in the next round of deals with Labour on the Welsh budget, and her party would be pressing the issue hard.
And Ms Williams said education would be a priority going into those talks.
"Why should it be that a pupil from the poorest background in Wrexham gets £450 extra for their education whereas if they were across the border in Cheshire, they'd get £1,500?" she added.
"That's an inequality. Welsh children are in danger of being left further behind because of the Welsh Labour government.
"Education used to be of the highest, highest standard in Wales - something we prided ourselves on - but unfortunately after a decade of Labour rule in Cardiff, we've slipped further behind.
"I want to change that and I want to start with the children who need the help the most".
So does she have a point? Maybe; But is comparing Wales to England as a whole a true comparison or even Wrexham with Cheshire?
Shouldn't comparisons be made with areas of similar economic and social deprivation .
How rather than England as a whole does Wales compare with the figures for English Regions.?
Ms Williams may well have a point , but can such problems solely be solved just by spending more money?
Are other factors, such as the parents attitude to their children's education something we need to work on?
If the parents are not interested or able to help their offspring outside school, then we are creating a circle of non-involvement that results in generations passing on their own poor education experience on to the next.
The Lib Dems make much of their Pupil Premium , but evidence of it success is thin on the ground and there is some disquiet that Schools are just adding it to their school budget and not allocating it where it is intended .
There is a strong argument that we need to increase expenditure on education in Wales, but the cake as grown smaller under the Tory/Lib Dems government and if opposition parties are to call for this maybe they should tell us who gets a smaller slice as a result.
But more importantly we need to ensure that any extra money goes where it is intended .
La Pasionaria got the headlines she wanted and is justified in pointing out that Labour who have run the Assembly alone or with the Lib Dems and Plaid and must take responsibility for the education failings here.
But apart from increasing spending what other solution does she, or any other Welsh politician have.