What would you consider appropriate for a basic class? I just became a TA (teaching assistant) for a basic class and had the same scenario with the students.
edit: this is a college class, 1800-2200
Actually, not only is the situation Rid mentioned inappropriate from a respect/professionalism point of view, but a practical manner.
In my program, you got kicked out if you wore anything except clean steel toe shoes.
My opinion? Black steel toe shoes, navy professional pants, class uniform shirt. All clothes clean and smooth, all hair short (or pulled up) and clean. No facial hair. No piercings. No hats. Well-kept.