If you want to describe a situation in which someone performs poorly on an exam, you could say that they "did not do well on the exam" or that they "underperformed on the exam." You could also say that they "did poorly on the exam" or that they "failed the exam." If you want to describe the cause of the poor performance, you could say that they "did not study enough" or that they "were not well-prepared for the exam."
