Muhammad Samiullah, an intermediate student, was arrested on Friday following a complaint from the Chief Controller of the Intermediate Board, who accused him of blasphemy.
A case of blasphemy was registered against him at the Shahrah-e-Noor Jahan police station.
The Chief Controller alleged that Samiullah, a resident of Mujtaba Colony in Malir Halt, violated the blasphemy law by writing "unacceptable remarks" against Prophet Mohammed in his answer sheets for three subjects.
Superintendent of Police Zafar Iqbal said the student was arrested following the complaint by the Chief Controller.
Police obtained all of Samiullah's answer sheets before taking him into custody.
A challan or charge-sheet was presented in court and Samiullah was sent to custody.
Samiullah also wrote a letter in which he admitted committing the mistake and sought forgiveness, Iqbal said.
Qudrat Sher Lodhi, an investigation officer for the case, told the Daily Times newspaper that Samiullah had told police that he was "brainwashed" by his cousins from Norway.
Lodhi said Samiullah had no physical or mental illness and that further investigation was underway.
The controversial blasphemy law has been at the centre of a heated debate since last year, with liberals and rights activists claiming that it is misused to settle personal scores and persecute minorities.
Punjab Governor Salmaan Taseer was gunned down earlier this month by a police guard who was angered by the politician's opposition to the blasphemy law.