Dress for work, you are a professional Driving Instructor so look like one. Too many instructors come to lessons looking very scruffy. Ok, maybe your pupil does not care what you wear (and this will certainly be true of some of them) but the mums and dads who recommend you do.
Dress smart and the rewards will come. One of the most successful things I have ever done is to wear my shirt and tie, albeit comedy ties for learners. This went towards helping me display that professional image. Now I am not dictating the use of a shirt and tie, in fact I have seen some very interesting stuff from a lady called Carol Tuttle who talks about dressing our truth where even the colours and materials we wear can help effect our outcomes etc. But definably smart and please clean.