I graduated May 15 of last year. I have not been in school since. Does this mean I can't get the student tax break?

No, you are considered a student in any year that you attended school for at least part of five months. 

Since you graduated half way through the fifth month of the year, you qualify as a student. 

Q. Does this mean I can't get the student tax break?

A. No.

The 5 month rule for being a full time student is frequently misconstrued.  There is no requirement, for you to have been a full time student, for 5 months, in order to claim a tuition credit.  You could be enrolled half time (or more) and drop out on the 2nd day of classes, and still qualify for the credit, if you actually paid tuition.

There are several tax attributes connected to being a full time student (e.g. kiddie tax, Retirement savers credit and more), but the biggest thing is whether you can be a "Qualifying Child" (QC) dependent.  The rule is "parts of 5 calendar months".  so, being enrolled, in school, full time, from Jan 31 to May 1 would technically qualify you as a QC dependent (if the other rules are met).