Simpelest and most complex running time solution, create a graph of all students and create vertexes of which ones should not be in the same section. Then try and colour the graph as best you can. If you try to colour the graph by descending order of the degree of their vertexes, you'll get a smaller amount of colours. Each colour represents a section. This method doesn't give the uber best results. There are other graph colouring schemes to produce the optimal number.

Update: as for limiting the amount of people with the same class/colour, you can do that progrematically. Once a colour is all used up, use a new one by force.