The rebuilding of Les Beaucamps High School, a £37m project, could start before the end of the year if the States of Guernsey agree.
The island's government is due to vote on funding for the two-year build in November.
Peter Le Cheminant, who has been the head teacher for 18 years, said the problems had just got worse.
He said: "There's always been problems with the building... they've been fixed up and then the problems start again."
Built in the 1950s, Les Beaucamps is the oldest of the three secondary schools built after World War II and currently has more than 500 pupils.