Quiet please!

Facilities management and supervisor is not a title that I necessarily want but have. We have a small library relative to the total school population. When more than one group wants to use the library, we have to make a choice. How disruptive will it be to have two groups in here? What resources does each need? Will students be able to demonstrate the restrain needed to be polite and respectful to each group? With limited resources and space, we have to try to be as accomodating to all those who ask.
But when something like this happens, we look like the bad guy. One teacher is using the library for a small group meeting and needs some quiet so as to conduct the meeting. At the same time, a class of students arrives, unannounced, to get copies of a novel checked out. They are noisy and oblivious to other activities going on or to the fact that they just entered a library and social graces tell you to be aware of the general "library voices" rule. Meanwhile, another student of walks through the chaos and announces, "I need a computer." Again, completely ignoring all of the rest of the activity just to take the iniative and further her own agenda. Then an originally scheduled class comes to the door. But they can't get in because the unannounced class is loitering in the doorway. So they wait and chat even though the doors are open. The small group discussing in the back now has stopped all their work to wait for the classes to leave. I have to stand up and take the typical librarian stance, shouting above the din, "Everyone needs to be quiet in the library. Please respect the others in this space. Be quiet or go outside. If you have the book you need, you may go back to class. I am sorry Ms. SOHOSO for the interruption. I am sure these students will be leaving shortly." Is it a statement of our society? Is this a temporary brain freeze in social manners? Is it possible for 120 students to work together in the same small space without disrupting the various activities going on? I wonder how things change when these same students go on to college. In the meantime, we all need to work in our small library so as to be able to accomodate as many as possible.