Congratulations on your new job!
I don’t think you have to wear pants all the time to get your students to take you seriously. However, you will need to focus on a few key aspects.
1. Hemline - Keep it just below the knee. It may look long at first, if you’re used to wearing skirts shorter, but it’s the professional standard.
2. Shape - I find pencil skirts more “grownup” than a-line. Stick to a straight cut, and skip the “flowier” styles.
3. Fit - Stay away from anything tight, especially if you can see bra straps or panty lines. You don’t want your students pondering your underwear while you’re teaching them about the Industrial Revolution.
You may also want to invest in a couple of black blazers, even if your dress code is casual. They’re professional, cute, and will cover up errant pit stains if you tend to get the nervous sweats.
Pick up a structured bag and some low black pumps and you’re ready to go!