Sparring with the Master

When people start sparring, in most schools it is rare that they will start sparring with the teacher of the school. However as time passes by, as long as the teacher is still actively sparring, they will get the opportunity to spar and learn from him. Chances are they will be lucky to even touch the teacher never mind hitting him at the start. But as the student progresses he/she will eventually be a tough match for the teacher and maybe the opportunity to get one over him will present itself. This could be a chance to throw the teacher perfectly, or to get a submission on him or maybe even a great opening for a perfect roundhouse kick. And as soon as the student sees this opening I GUARANTEE that a small thought will pop up saying "shall I do it or shall I not as its the teacher, I don't want to embarrass him by beating him so maybe I should hold back" A lot of martial artists I have spoke to have given me different opinions on this. Some would hold back while others see it as a way of proving to themselves that they are progressing. Personally I think it depends on the teacher. If the teacher is one of the show off sorts then maybe a taste of his own medicine will shock him into being more humble. Then again, I think that a student should always have respect for his/her teacher. Any views?