Islamabad, Pakistan (CNN) -- Pakistan must immediately drop blasphemy charges against a teenager and let him out of jail, Human Rights Watch said.

"Pakistan has set the standard for intolerance when it comes to misusing blasphemy laws, but sending a schoolboy to jail for something he scribbled on an exam paper is truly appalling," Bede Sheppard, senior children's rights researcher at Human Rights Watch, said in a statement Wednesday.

Police told CNN Tuesday that they had arrested a teenager accused of writing insulting comments about Muslim prophet Mohammed in a school exam.

They arrested 17 year-old Sami Ullah in Karachi after receiving a complaint from the local board of education, said Karachi police official Qudrat Shah Lodhi.

Lodhi declined to say what Ullah wrote in his high school exam for fear of violating Pakistan's blasphemy laws himself.

Human Rights Watch argued Wednesday that the Convention on the Rights of the Child guarantees freedom of thought and conscience to anyone under 18. Pakistan is a signatory to the international treaty, the campaigners said.

It said Pakistan has applied its blasphemy laws against children before.

The controversial laws say whoever defiles the name of the prophet shall be punished by death or imprisoned for life.

The laws came into sharp focus early this year when liberal Pakistani politician Salman Taseer was gunned down after leading a public campaign to change them.

Taseer said the laws were being misused to persecute minorities.

Ullah - the latest Pakistani to be accused of blasphemy - is a Muslim, according to police.

He wrote the alleged blasphemous comments when taking a school exam in April 2010, police said.

It's not clear why it took months for the board of education's complaint against Ullah to reach police.

Ullah has apologized for his comments and begged the court for leniency, police said.

CNN's Reza Sayah and Journalist Nasir Habib contrinuted to this report