I've never heard of a school that has students on LA rotations wear professional dress, here I'm pretty sure it's khakis and a polo or coveralls, depending on the rotation/what you're doing. SA rotations have a more strict dress code, but I usually see the students in slacks, a nice blouse and flats, not a skirt and heels. And I expect LA/mixed vets would still dress professionally if they were attending a conference, CE lecture, etc.
I wish we didn't have a dress code for lectures, but it kind of makes sense if we have a speaker in class or you just want to go up and ask a professor a question after class, it's nice to make a good impression. You never know who you might end up needing a letter of recommendation from or wanting to work with down the line. Also like someone else said, we sometimes have donors or alumni touring the facilities, so it looks a lot better to them to see students dressed nicely. If we're on campus after 5pm, nobody cares what you wear. Our dress code for lectures isn't super strict, the main things not allowed are T-shirts, sweatpants, yoga pants, leggings, short shorts and tennis shoes (jeans are allowed). It's also enforced more by our class officers than the faculty, I would be extremely surprised if anyone got formally disciplined just for dress code.