Insurance companies not allowing students

[Jon Carter]

Hello all,
I just purchased my third rental property this year, a 3 bed 2 bath half duplex, and called my usual insurance company for another policy. They told me that they wouldn't insure me if I was going to rent to students. This wasn't even a question with my first two properties, but after a few more calls to other insurance agencies AND going through a broker, I found that ALL were demanding the same policy. Has anyone else had this problem?
It seems to me as being such a grey area as to what constitutes a 'student.' If a parent decides to go back to school in his/her fifties, are they now classed as a student? I know this isn't what the insurance companies intend to mean when they say someone is a student, but it seems to be such a grey area if the time comes when I may need to claim. Thanks, and any advice would be greatly appreciated.
Jon