There were actually two strikes on two separate schools that day, one in Minab and one in Lamerd. The one in Minab that killed all the schoolgirls overshadowed the coverage of the other (I'm assuming because of the moral outrage of killing so many schoolgirls). I didn't actually know about the second until NYT did a report about it much later (linked above).

You know we double tapped that one school, right? an hour after the first bomb the school was full of emergency responders and parents searching for their kids and a second bomb hit while that was happening.