The National Association of Head Teachers, at its annual conference in Harrogate, demanded ministers provide funding to ensure that children are taught in reasonably-sized classes and in schools in good repair.
But they stopped short of supporting industrial action currently being considered by the three big classroom unions in protest at rising class numbers.
Instead, the union has decided to take a tough line by warning teachers who take industrial action over class size that they could face loss of salary or ultimately dismissal.
General secretary David Hart rejected a suggestion by the National Association of Schoolmasters Union of Women Teachers that children in over-sized classes could be excluded by rota and sent to sit outside the head's office. He said if some children were excluded, then the whole class would have to be sent home.
While the union has every sympathy for teachers coping with ever-growing classes, heads - as managers - must warn staff when they are in breach of contract. Mr Hart said he thought any action based upon workload would be questionable.
In its advice to members, the NAHT says that governing bodies and LEAs must not allow volunteer parents to take lessons during a dispute. Governing bodies will have to decide what disciplinary action to take, and this could include withholding salary, suspension without pay, and dismissal. Heads must provide the governing body with the names of those involved in industrial action if requested.
Delegates overwhelmingly supported a motion which said that lower class size was significant in improving educational performance. Speakers asked why, if ministers did not believe this, did they send their children to independent schools which sold themselves on having small classes.
The attack on Government funding levels was led by Jackie Thorp, of Yew Tree school, Chadderton, Manchester, who said education cuts had irreversible effects.
"It is this Government that has introduced the national curriculum, assessment and the special needs code of practice, and it should be this Government that ensures the necessary funding to meet their costs instead of wasting millions of pounds on ill-thought-out initiatives, charters for everyone except teachers, glossy brochures that no one reads or requires, unaccountable quangos and the gross inequities of grant-maintained schools and city technology colleges," she said.
More than 4,000 teacher redundancies have already been identified because of this year's budget cuts, according to an NAHT survey published to coincide with the conference. The survey, of 270 primary and secondary schools from 47 local education authorities, has revealed rising classes, and that some of the least able children are being hardest hit by cuts.
More than two-thirds of schools reported pupil:teacher ratios worsening by an average 2.5 per cent between 199394 and 199495, and 67 per cent said they would not be able to meet the statutory requirements for pupils who have special needs, but who have not been statemented.
A third of schools also said they could not meet the requirements for pupils with statements.
Learning support assistants and small groups for special needs pupils were likely to be most at risk. Mr Hart said the cuts made a mockery of the Government's code of practice, designed to ensure provision for special needs pupils.
The survey also showed that more than half of the schools felt their ability to meet national curriculum requirements would be diminished this year.
And although 54 per cent said their budgets had increased, the NAHT said that actually represented a standstill or decrease in real terms, taking into account the teachers' pay award, inflation and rising pupil numbers.
Mr Hart said cash decreases in 46 per cent of schools meant very severe cuts.
Delegates to the Harrogate conference heard that there would be an extra 120,000 pupils in September, and that a third of education authorities would be unable to fund them. Inflation had not been fully funded by 71 per cent of LEAs and for the past two years up to half had been unable to fund pay awards in full or in part.
Mrs Thorp said: "This Government parrots on about the lack of research to support the view that smaller class sizes result in higher standards of teaching and learning. At the same time it also refuses to finance such research.
"The simple truth is that it doesn't suit this Government's priorities to provide for the children of their constituents what they provide for their own."