What type of phrase or clause is "was attending"

The correct answer is: the verb phrase.

Explanation:

The verb phrase consists of a main verb alone, or the main verb + any auxiliary or modal verbs.

In this case, the verb phrase was attending has a continuous meaning. It's a syntactic unit composed of at least one verb and its dependents (objects, complements and other modifiers), and not always includes the subject.

The verb phrase is similar to what is considered a predicate.