It was. He belonged to that ROTC last year, before he was expelled from that school to attend another. When you leave they collect your 'issued' uniform but you are allowed to keep person purchased stuff, such as the Polo Shirt he was wearing. He no doubt did this to blend in with students to allow his escape.
Ironically enough, it was one of his RTOC buddies that recognozed him earlier (in the day) and then called LE after they (the ROTC class) heard the shots fired and retreated to their classrom.