Student registration fee at conferences for ex-students for work done as a student
You're not a student now

The fees are based on the status of the participants, not the status of the papers. Often, organizers will require some evidence of student status (such as a current student ID) in order to be eligible for the reduced price.

It may be reasonable to get the student fee if you're a student at the time of registration but graduate before the actual conference - as the process often takes multiple months, but if it's something earlier, then you're out of luck.