Is this your first semester of nursing school
? That sounds like something a Fundamentals instructor would do. If a patient says no, then you can't give them a bath. You could always try to convince them, but with some people, it's just not going to happen. One thing I do with patients to help them bathe is instead of asking, "Would you like to bathe today?" is saying, "It's time to get cleaned up. What do you need help with?" This way, it promotes independence, and if they are just adamant about it, they can refuse.
I think most instructors are like that because a lot of students try getting out of giving a bath. It's not really fair when the patient refuses though. Sometimes you can convince them to get a bed change, and get them a warm washcloth to wipe off their face. Maybe that will help.
I used to be really nervous about giving baths my first semester, but after working as a tech in the summer, you just get used to asking.
Good luck with nursing school!