Students of Karabar High School were told to stay home on Tuesday after a burst water main forced the school to close.
Parents were informed of the closure in an announcement from the school on Monday afternoon, which was meant to be the students' first day back at school for term three classes.
While year 7 to 11 students were told to stay home, trial HSC exams for year 12 students were still being held at the school.
A NSW education department spokesman said the school would be fully operational on Wednesday and would be open to all students.
A spokesman for Queanbeyan Palerang Regional Council said the water issues were caused by a main on the campus that only supplied water to the school.
Nearby homes and businesses were not affected.