Final answer:
The phrase 'I am not used to [sleeping] in on the weekends' is a gerund phrase that functions as the object of the preposition 'to' and represents the student's lack of habit or regularity of sleeping in on the weekends.
Step-by-step explanation:
The phrase 'I am not used to [sleeping] in on the weekends' is a gerund phrase. A gerund is a verb form that ends in -ing and functions as a noun. In this case, the gerund phrase is functioning as the object of the preposition 'to' and it shows the student's lack of habit or regularity of sleeping in on the weekends.