Is last week a noun

No, the term 'last week' is a noun phrase, made up of the adjective 'last' describing the noun 'week'.

The noun phrase 'last week' can function as a subject or an object in a sentence.

Example: Last week was the last week of school. (both subject and direct object in this sentence)

A pronoun is a Word that takes the place of a noun in a sentence. The pronoun that takes the place of the noun phrase 'last week' is it.

Example: Last week was very busy. In fact it was quite chaotic. (the pronoun 'it' takes the place of the noun phrase 'last week' in the second sentence)