The Arun St school has needed a new classroom since 2015, and has been forced to take measures to squeeze in a growing number of pupils.
In 2016, a third of the school's library was walled off to create a temporary classroom when it split its new-entrants class in half.
The library is now upstairs in the school hall and a meeting room, also in the hall, has been converted into a classroom which is also used for the school's after-school programme.
Fenwick School principal Rodney McLellan said the school had to be inventive when it came to squeezing in all of its 325 pupils.
"We can fit them in ... but it's a case of being creative with the space that we have got.
"We will move junior classes to that space (former meeting room) during term four, the new entrants."
He said the Ministry of Education recognised the school's plight at regional level
but "we're just battling in that pool of nationwide needs in terms of school building replacements and additional classroom spaces".
"They have a limited pool of money and they have to put buildings where they best suit."
He said until an additional classroom was constructed or moved to the school, frustration would remain.
"What we are operating in terms of a library is not what we would like to have in terms of a space for our children. We have got a space that does work well as a classroom ... but we prefer that to be our library and have another space somewhere to use as a classroom.
"There's a bit of frustration, but we know we have done what we can to put our case forward."
