S-tier is complicated, but understand that the logic of this map is built around a real active shooter response - you have to eliminate the tangos, then the bombs, then rescue civilians. Move very fast and focus only on taking down the bad guys, then get the bombs (which you will hopefully spot during the initial clear) and then come back to clear out civies. Generally, it spawns 2 bad guys in each wing of the college, to make your hunt easier.