The teacher at West Denton High School, in Newcastle upon Tyne, set the practical chemistry paper on May 21 rather than June 14.
Now more than 500 pupils at 59 schools will have to take a different test - although the seven pupils at West Denton have been told they will not have to retake it.
Examiners at the University of Oxford Delegacy discovered the error when papers arrived for marking. Next year's paper has been finished early and distributed as an emergency replacement.
Jennifer Slater, headteacher at the 600-pupil school, said: "This is a serious mistake with serious consequences, but our exam programme is going ahead and our candidates will not be affected.
"The teacher involved has been suspended, not as a punishment but pending an inquiry, which is the usual procedure."