Will I be considered a full-time student in 2021?

Student defined. To qualify as a student, your child must be, during some part of each of any 5 calendar months of the year:
1. A full-time student at a school that has a
regular teaching staff, course of study, and
a regularly enrolled student body at the
school. 

Given the IRS definition above could you argue that you were still enrolled in the school through graduation?  Your decision but I think you could make a case for this. 

Yes, the graduation ceremony occurring in May counts as attendance in May for purposes of the 5 month rule.

Yes, you would be considered a full-time student if you graduated in May. 

According to Publication 17of the IRS, you are a full-time student if you are enrolled at a school for the number of hours or classes that the school considers full time. You must be full-time for some part of each of 5 calendar months during the year. You would still be a full time student  before the May graduation. Bsch4477 and Hal_AI  are absolutely correct.