The school is one of the biggest in the region and currently has room for 308 pupils, but has 306 on the roll for August.
And now education officials have revealed plans to increase capacity to 368, at a cost of £20,000 a year for the temporary accommodation, plus £10,000 to set it up.
However, if the roll continues to increase, they will consider building a new classroom or even a whole new school for approximately £14 million.
With new homes in the pipeline for the area, council officers say they will be keeping a close eye on future enrolment numbers and take action if and when needs be.
In a report on the situation for councillors, education officer Laran Foss explained more: “Should the projected school roll for Gretna Primary School be above the capacity, 308, then temporary accommodation would be procured and sited in proximity to the main building, to allow for shared use of the associated spaces such as dining facilities. Toilets will be included within the temporary accommodation.
“In the first instance a double classroom unit would be procured to release a general purpose classroom for the school, which has currently been used as a teaching space. The additional classroom unit would consist of two class bases thus increasing the school capacity by 60, to 368.”
He added: “Should the number of pupils prove stable over a number of years then the temporary accommodation would be removed and a permanent solution put in place.
“If pupil numbers do not stabilise and continue to grow beyond the additional 60 places, then there is the option to add one further classroom on a temporary basis, taking the school capacity up to 398. It would not be feasible to go beyond this capacity as the non-teaching accommodation would be unable to cater for the number of pupils.”
In that situation, the council would have to consider a “new or replacement school”, estimated to cost £13.9 million.
Members of the council’s education committee will next week be asked to agree to the temporary classroom. Committee chairman Councillor Jeff Leaver said: “We know from some of our major projects over recent years, such as St Joseph’s College in Dumfries, that the temporary accommodation was of high quality and provided a stimulating learning environment.
“I am pleased that councillors will be presented with options that will address the situation at Gretna and will provide a good solution for both our staff and pupils.”