Parents at an overcrowded school where children are taught in the corridor are blaming their county council for the cramped conditions.
Pupil numbers at West Green Primary in Crawley have shot up from 104 to 172 since its last inspection in 2005.
Two year groups are taught together in a single class of 40 pupils to try to cope with the situation, with some children being taught in an alcove in a corridor.
But school governors say West Sussex County Council is refusing to provide extra space until numbers hit the 180 mark.
They appealed to parents to support a letter-writing campaign to the council and the Government to try to get an extension to the Fifties buildings.
Parent governor Andrew Coleman told The Argus: "It is disappointing we have had to resort to this to try to get the council to listen to us."
The problem started when the school changed from a "first" school - taking children up to the age of nine - to a primary school, with children staying on until they are aged 11.
Before that, older children would spend two years at another primary school before moving on to secondary education.
The number of applications for admission rose when the year groups changed. Mr Coleman said: "I don't think the council realised how successful it was going to be.
"We began to run out of space."
The school has only six classrooms for seven year groups.
West Green's last Ofsted report in 2005 said pupils, one in five of whom do not speak English as their first language, progressed well and showed "good" achievement.
Inspectors praised the school's leadership and management.
Headteacher Julie Dowling said: "Although plans have been developed to extend the school, we do not have a definite start date for these improvements and we face another year in inadequate accommodation and with a temporary classroom."
A spokeswoman for West Sussex County Council said the local authority would provide a new classroom in September.
The council would avoid using huts for temporary classrooms "if at all possible".
The spokeswoman added: "The county council is currently working with the school to provide a seventh class base and additional facilities to support the school's infrastructure.
"This additional classroom will be an interim arrangement while the design of the permanent solution takes place for a start on site later this year."
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