To be a full fledged teacher and open up a dojo no. But to be hired by a dojo to teach people they feel you are qualified to teach then yes perhaps.
In my school it's taught that as a higher ranking belt you are an idol and guide to those belts below you. An instructor should only be bothered with a question if you ask someone in your rank or a higher belt rank and they can not provide the correct answer.
As you train you should not only be looking out for yourself but for others, if you see someones kicking technique is wrong you dont ignore it and wait for the teacher to get around to correcting them you say -hey this is what you should do instead- because it's what you would want someone to do for you in return before being criticized by your instructor. You could also get more praise for looking like you knew what you where doing the whole time lol =]
We have student instructors but they usually teach younger students and have it as their part time job while the actual instructors teach the big stuff. During a class an instructor might ask one student or two to show others how to preform something, help them, or lead in a basic exercise because he knows that/those students are capable... nothing is asked of you that you aren't able to do, the instructors do their jobs but sometimes it's necessary for others to step up to keep things flowing more quickly and not hold up the whole class.
It's not weather you can or can't do something. It's weather you will or won't.
~*~If I'm here digging around for extended periods of time I'm just avoiding work =P~*~