He said the union welcomed the Special Educational Needs and Disability Bill set out in the Queen's Speech - but change needed to be planned, taking account of that inclusion agenda.
The number of children with "statements" setting out the help those with special educational needs require - and triggering funding for it - has risen by an average of 16.7% over the last three years, the Secondary Heads Association says.
The union's general secretary, John Dunford, said this had "considerable funding implications" for schools whose budgets were already over-stretched.
Changing emphasis
The money the government did provide was not keeping pace with the increasing demand, he said, particularly with the shift towards greater "inclusion", educating children with special needs in mainstream schools.
He said the union welcomed the Special Educational Needs and Disability Bill set out in the Queen's Speech - but change needed to be planned, taking account of that inclusion agenda.
Geoff Price, head of Warwick Road Special School in Bishop Auckland, Co Durham, said: "We must ensure that inclusion is phased and paced so as to be manageable for schools.
"With adequate resources and a realistic timetable we can establish successful practice.
"But there is a danger that we overload the process of change by trying to do too much too soon."
More money, new code
The Department for Education said the increase in the number of pupils with statements in England reflected the overall rise in the number of children attending secondary schools.
The total percentage had increased only slightly, from 2.3% to 2.5%, since January 1997, said a spokesman.
Cash to help them would rise by £27m to £82m next year and the government was providing £220m to schools to improve access for disabled people.
Earlier this week, the Education Secretary, David Blunkett, published a draft revised code of practice aimed at identifying special needs earlier, getting parents more closely involved, and taking into account the views of the children themselves.
But critics have claimed this weakens the protection a "statement" can give, by removing the current requirement for the level of help to be quantified - so it will be harder to object to cuts in children's specialist teaching or therapy.
A final version of the code is to be published once the bill has gone through Parliament.
The union's survey involved questionnaires returned by 450 schools.