Difference between Termly Attendance

I'm doing some work for a school and want to be able to show the difference in attendance between academic terms. I have a date table set up and I have allocated a Term (1,2,3,4,5,6) to each date based upon a look up from another table that contains the term dates. I have added columns such as month/day/year etc, so I can use the PREVIOUSDAY/Month/Year functions, but I can't fathom a way to compare one term to another.

Any help would be appreciated.

Thanks

Jon