When I was at uni, I could freely attend lectures of classes I hadn't taken, even from different faculties.

I was wondering, if it's possible for Hogwarts students to attend some lectures of elective classes, even if they don't take the class?

For example, I have Tony. He likely won't take Muggle Studies as he is a muggleborn and he'd prefer to take a class he'd feel he'd learn something new. Still, he might have been interested in attending a class of Muggle Studies if it was going to mention something that interests him, and he had the free time.

Perhaps this isn't done in all classes, or you need special permission from the Professor that teaches it to attend. What do you think?