Set high standards in yourself, your training room and with your students, of course. Set a high professional standard that your students will aspire to.
The next question is: What sort of atmosphere do you want to create? Formal; informal; serious; light-hearted?
Clearly much will depend on the nature of what is being taught.
Public Speaking and self-development course should best be delivered in a friendly and supportive environment.
Clinical procedures or material with legal implications needs a more fixed, and formal environment.
To produce the correct learning environment, the trainer has two suggested tools at hand: (i.) His or her own demeanour and attitude (ii.) The physical layout of the room.
Focussing primarily here on (i.), the trainer can greatly influence his learning environment by tone of voice, physical demeanour and also by dress.
As a participant, coming into a room where the trainer is wearing jeans and a T shirt, there is going be very different expectations to entering a room where a trainer is wearing a business suit.
Therefore, it's absolutely vital that we ensure we are dressed appropriately. Male trainers these days need to consider whether a tie should be worn or not. As a rule of thumb, if unsure how to dress, it will always be better to dress too formally rather than under-dress. Clearly, arriving at a training session with a suit and tie and finding everyone else informally dressed, requires no more than taking off the tie and jacket - and everything is fine. On the other hand, you may look a bit silly putting a tie on in front of everyone if you have arrived under-dressed! (I suppose it is better to give an impression of being over-prepared rather than under-prepared!)
In truth, as a 'teacher', your students would possibly expect you to be more formally dressed than they are. You should also consider your personal and natural style of delivery which has a major impact on your students. Because of my nature, soft and somewhat unassuming, I never believed that I came across in severe manner. In the initial days of my training practice, a more formal dress code helped me produce an increased level of authority. On the other hand, I was aware of colleagues whose natural communication style was already fairly strong and authoritarian and so it helped them sometimes to dress down a little to remove some of their sharp edges.
So the key to pitching your training at the right level is to establish what sort of environment you need to establish to help your participants learn best. And then decide what you need to focus on, based on your own natural style and attributes, your appearance and attitude, to bring that environment into being.