In the GuP universe, where teenage girls with high emotions are given access to such weaponry, it only makes sense that many of their most controversial/important buildings would have such a defense. Tsuji Renta's office most likely has several layers of it. It's probably the only reason why Shiho didn't storm his office with a tank in the movie. (ツ)
Oh man, we've really arrived...Acknowledge our resolve! Fire!!*BAM*!AAAAAAAH! Our first criminal offence!!You didn't invite us to the Spring High Volleyball...Damnit!! The wall of the Spring High is physically thick after all!!Th-This is...Carbon coating!?*PLINK**BAM*Huh!? That's what they mean by the wall!?