Science labs at La Mare de Carteret School have been closed after a health and safety review.
Three prohibition notices were issued because of gas and electric supplies as well as the storage of chemicals.
'Essential' works must now take place, they say.
That has been scheduled for the summer holidays with made given to the school by the States after a debate in March.
The works will be completed by September, Guernsey States say.
The school had been described as being 'in desperate need' of long term investment, although some interim work has been taking place.