People from the student union were responsible for maintaining order outside the library. Of course, they understood that it was nothing more than the reaction of collided spells, but the issue was that it was a freshman test on that day. How could a freshman execute magic of such a level?!
The student union was actually in a very awkward spot during the assessment. They had plenty of authority in previous years' assessments, but the military's powerful intervention had reduced them to being responsible for maintaining order at the venue. Furthermore, they were still in front of the freshmen, so they had to maintain seriousness and could not let their grievances show. That way, the student union's authority could still be preserved.