Step 1: Identify the first subject.
Two subjects are joined by as well as: The Principal and teachers. When subjects are joined by as well as, the verb agrees with the first subject only.
Step 2: Apply the rule.
The Principal is singular, so the verb must be singular: was present, not were present. Part B uses were, which is plural and incorrect.
Step 3: State the rule.
A as well as B always takes a verb that agrees with A alone, not B. Part B is the error.
\[ \boxed{\text{were present at}} \]