Parents and councillors have demanded action over a school which has had temporary classrooms for 30 years.
They say education at Wivelsfield Primary School, near Burgess Hill, is suffering.
Two main classrooms and three temporary classrooms accommodate 118 pupils, aged from five to 11. The school has to rent a hall and playing field from nearby landowners and a church.
Now Wivelsfield Parish Council chairman Martyn Long has written to East Sussex County Council, expressing his disgust at its failure to take action.
He said: "It's disgraceful that, after 30 years, we are still waiting.
"The school is Victorian. Academically, the school is fine. But it falls down on its facilities.
"It must be one of the worst schools in East Sussex and yet the council has replaced other schools built in the Seventies."
Mr Long said developers wanting to build houses in the area had agreed to build a new school in return for planning permission. This was rejected by Lewes District Council, which did not want houses on the site.
Two temporary classrooms were installed in the Seventies. Both were replaced by larger cabins in the Nineties, said Mr Long.
He said: "It's total incompetence as far as the county council is concerned. They have refused a meeting and now totally ignore us. Temporary classrooms should be just that - temporary."
Head teacher Rita Tipple said: "It is a very charming school from the outside. It is Victorian, very pretty and in the country. But it is not really appropriate for the modern curriculum.
"The county council has not identified a site for a new school nor has it any funds to provide one.
"We feel the present site is too small. How long is a temporary classroom going to remain temporary?"
A county council spokesman said: "Unfortunately we have a very restricted budget. We have to make priorities and we have to look at the county as a whole."
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