School tries to make me cover during last hr prep

The school I work at never asked me to cover for a last period class officially. They scattered notes around for certain subs to cover….do they have an actual record of who subs in what class during their prep as this it’s not the official class I am supposed to sub for that’s on the record. I do not want to stay for last hr of school if I don’t have to cuz I am allowed to leave during last period prep otherwise….i don’t get paid extra for covering the class and it’s not in my official schedule.