Final answer:
The italicized portion of the sentence 'headed by a senior' is identified as a past participial phrase which is providing additional information about the noun 'the group' by describing its leadership.
Step-by-step explanation:
The italicized portion of the sentence 'headed by a senior, the group drew up rules for school spirit week' is a past participial phrase. This grammatical structure uses a verb in its past participle form to modify a noun or pronoun, in this case, 'the group.'
A past participial phrase typically provides additional information about the noun or pronoun it modifies, such as its state or condition, and in the given sentence, it describes the group as being led by a senior. This is different from a present participial phrase, which would involve a verb in its '-ing' form, or an appositive, which is a noun or noun phrase that renames another noun right beside it.
In this example, 'headed' is the past participle of the verb 'head,' 'by a senior' is a prepositional phrase within the participial phrase, and the entire phrase acts as an adjective, providing more information about the subject 'the group.'