At my school (my daughters) there is one aide. This aide does float between different tasks in multiple rooms. Next year they may have to have another aide, but the way it worked this last year was that she was scheduled to be in certain rooms with certain students during set periods of time. So, like from 10am to 10:45 she is with A in her classroom, helping her work on such and such... then she has to go to B's class to help supervised him while they do an art project... after that she goes and takes charge of C, taking him to lunch, changing him after that, and taking him out to the playground to swing until such and such time when she returns him to his teacher and goes to class 3 to help in general..
you know what I mean? it was set schedule for each day. When the aide had to be two places at once, such as general helping class 3 (pre-k class) and being with student A, sometimes they have student A go to class 3 to do their work, where the aide can help both A, and those around A.
This is what I have observed. Occasionally my daughter wanted me to stay the whole day, so I helped out then in her class, which was a help to the aide as she could leave the class from time to time to get prep work done for when she had to be with the other students not in the pre-k class. (The pre-k class teacher is also a 6 and under spec. ed teacher, thus why the aide, some of those students where under IEP's, and it was a mixed/main stream class.)