Answer:
,the principal at my school and the oldest staff member,
Explanation:
Appossitives are words or phrases that define or rename the noun that they follow. Besides, if they are non-restrictive clauses, they are usually separated from the noun and the rest of the sentence by commas, dashes or parenthesis. Thus, in the sentence given, the appositive phrase written within commas specifies who Mr. Dirksen is.