KABUL—A blast that killed scores of students preparing here for their university entrance examinations capped an extraordinary week of violence in Afghanistan, undermining what the U.S. and Afghan governments had considered real prospects for an imminent cease-fire and a new round of peace talks with the Taliban.
Afghanistan’s Health Ministry said at least 48 people were killed and another 67 wounded in the capital on Wednesday when a man wearing an explosives vest blew himself up in a classroom full of students. The blast...