Well, it all depends on your profession. I expect that the avatar of an SL artist would look different than a corporative avatar.
But in any case, I think that SL allows more freedom than RL. Of course it has its own conventions, its own code, that sometimes differ from RL ones, and one should know them and respect them, or at least but be aware of them.
A basic avatar as a teacher? Please no!!
Newbies have a particular look, and they move in a particular way. We all feel a sort of tenderness towards newbies: they remind us of our childhood, of the problems we had, and we are very willing to help (and we are more tolerant and pacient), but somehow… we do not really trust them.
There is nothing bad in looking like a newbie, but when taking a class we usually want to know that our teacher is in charge of the situation. That happens in RL and in SL alike. We are there, ready to follow him/she/it, and you agree with me that nobody wants to be guided by someone who has just landed in this world.
I know that one can be a wonderful teacher and very skilled in SL, and still look like the first day, but the first impression will be.. disappointing. It would be like entering a class and seing that your teacher is 16. May be he’s a little genious, but he will have to work very hard to win your confidence.
Teachers should work on their avatar, that can be green with pink hairs, but will never, never be looking like a Ruth.