How would an empty magazine be considered a weapon?

My friends and me go shooting on the weekends. My friend lost one of the mags to his 9mm Beretta and lo and behold, he finds it in his coat pocket when he gets to school, and is promptly suspended for "weapons on school grounds". I declare BS because it was an empty mag that was accidentally left in a pocket. Not like he meant to bring it and not like it was the Beretta the mag goes to. How is that a weapon?