Hi Natalie, here is my experience. I entered a class as a teacher, as opposed to a student, for the first time at the age of 23, some of my students being older than me. And I did look younger than my real age. I was very worried about what to wear, I had to tame a class of 130 students, and I did want to look professional and authoritative. I chose a ridiculous out-of-fashion over-feminine miss-the-teacher-with-the-red-pencil outfit, and I honestly looked like I was wearing my mother's clothes (and make-up and jewelery!). The students still mistook me for one of them, and moreover I was very uncomfortable in this outfit which was so much not me. The following year I wore my usual jeans and shoes, with a touch of grown-upness (a pretty shirt, a very plain necklace). Some of the students still couldn't believe I was the teacher, but it didn't last for long. And, being comfortable in my clothes, I quickly gained the confidence which finally gave me the teacher look which I was looking for. I still have pretty much the same childish features, but I do not worry about it anymore: it's very cheesy but, yes, authority is something that comes from inside, and you might as well be comfortable. Summing up: I would add a touch of elegance to your usual outfit, but I definitely wouldn't overdo. And, more important of all, I would choose a style that represents you, and not something that is completely not you. It will help you to overcome the initial shyness and awkwardness.