At its annual conference in London, the union called for a "fairer funding system" for schools.
Ministers say tackling deprivation is at the heart of their funding policy.
The ASCL analysed figures on schools in England from the Department for Education and Skills by looking at the numbers of children receiving free school meals and levels of funding.
It found that 25% of schools with low deprivation - classed as having between zero and 8% of pupils receiving free meals - receive nearly the same amount as 25% of schools with high deprivation.
Related articles:
