Schoolchildren sat an English Literature GCSE with no knowledge of the some of the texts after spending a year studying parts of the wrong syllabus.
A group of ten pupils at Blatchington Mill School and Sixth Form College, in Brighton and Hove, sat the exam on May 22.
A parent, who wished to remain anonymous, said: “My child came out of the exam and went mad, saying they had had no idea what was in some of the exam paper.
“All the others who had sat the exam said the same.”
She said parents were sent an email from the teacher of the class admitting to the mistake and apologising.
The parent, who described the error as “inexcusable”, said: “They were supposed to compare an unseen poem to three they knew.
“Instead they saw four unseen poems.”
Headteacher Janet Felkin said: “We went immediately to the exam board who are taking their own steps.
“Mistakes do happen. We are taking action and it won’t happen again.”
A spokeswoman for assessment board AQA said: “We are currently working with Blatchington Mill School to look into this matter.
“In circumstances where students have studied the wrong text by mistake, we work to find the most appropriate solution so that they are not disadvantaged.”
She added that there are a number of allowances that can be applied under what are known as “special considerations” but at this stage it is too early to say what these could be.
A spokeswoman for the school said: “Blatchington Mill School investigates and communicates thoroughly if an anomaly of any nature is reported.
“We also ensure that all parties involved with any such instances receive full information directly.
“Our staff are available to answer any queries from parents and students regarding school matters and welcome the opportunity to allay any concerns.”
The Joint Council for Qualifications guide to the special consideration process states: “Centres are advised that it is their responsibility to ensure that the correct texts are taught.
“Where this has not happened there can be no guarantee that a candidate will receive special consideration.
“Such incidences will be investigated by the awarding body on a case-by-case basis.”