Seriously, a good instructor should never ask a student to do more than they are truly capable of. The reason for this is that to do is to invite failure. Once you have someone failing it is hard to get them to look at the positive. Human nature is to always concentrate on the negative, which is why agood instructor should try to push their students as far as possible, whilst recognising how far that is.
Tonight I had a 41-year old, who had never taken formal lessons, and only attended one previous class doing jumping side kicks. In fact, I had the whole of the class doing them regardless of grade, because I knew that they could do them and the other kicks I had them doing. All of them rose to the occasion and did me proud. That, as an instructor is all I want. Just try your hardest every time.