I instructed at a flying school for 2 years before I got a, ehem, real job. We all wore white airline shirts with black trousers and NO stripes. It looked professional without looking conceited. Seemed to impress the RAF as we hed a Flying Scholarship approval that required revalidating every year. BTW it was the hardest type of flying work I have ever done. Despite what a lot of none-instructors say, it is still one of the best ways to get in the right seat of something a little bigger!