The school crowd is far more interested in MacBooks as far as I know.
MacBooks are the hot thing for the students, because they take them to and from class.
I doubt many college students are rushing out to buy huge iMacs, especially as they are moving them from the dorm at the end of each semester.
Apple, being not stupid, knows this and I would think a very small proportion of iMac customers are college students, or any kind of student.
I doubt the planning of iMac releases has anything to do with the school schedule, MacBooks would have a much higher chance of being dependant on that.