In “The Office,” Michael graduated in the same high school class as Phyllis Lapin. Phyllis brings this up when Michael patronizes her as though she were much older, reminding him that they are the same age and were high-school classmates.
The joke works because Michael repeatedly treats Phyllis as elderly, calling her “like a grandmother” and making comments about her appearance. Phyllis’s calm correction punctures his attempted insult: despite their very different personalities, office roles, and appearances, they are peers from the same graduating class.
A common mix-up comes from later continuity. Michael says he struggled with second grade and eventually “aced” it, implying he may have been held back. That could make him older than Phyllis even if they graduated together, but it does not change the show’s direct answer. The series consistently identifies Phyllis as Michael’s high-school classmate, not Pam, Angela, or one of his other Dunder Mifflin colleagues.