There's no readily available, definitive data source that calculates the number of colleges per square mile for every US state. Such a calculation would require precise geographic boundaries for each college campus and very precise state area data, which isn't consistently compiled in a publicly accessible way.
While you might find lists ranking states by total number of colleges, that doesn't account for land area. Highly populated, small states might appear to have a high density, but lack the overall total number of institutions found in larger states. Therefore, there's no simple answer to this question.