Endan Genpei Souranki: Irohanihoheto

The main character is your average school student. He sleeps during the history class and gives the wrong answer on the question. In the night he slips into the school to correct the mistake, but the answer is already input in the computer. When he messes with the computer he arrives to 1169 year, the events before the Genpei War. If he can't fix the answer, he will change history instead to fit the answer!