MARDAN, Pakistan  A teenage suicide bomber walked onto the parade ground of a major military training school in northwest Pakistan on Thursday and blew himself up, killing 27 cadets, officials said.

The attack at the Punjab Regimental Center in Mardan was the second by militants against the school in the last three years.

The attacker on Thursday, a 15-year-old boy, chose the morning parade lineup to inflict maximum casualties, an army colonel said. He was dressed in the uniform of a civilian school near the military academy compound.