These are great questions for teachers to keep in mind.
Currently, I am teaching a Year 10 English class (Grade 9 in America) and focusing on a film study.
This am I working at the right level? question, combined with the need for differentiation, makes this a very tricky proposition.
Teacher knowledge, experience, and professional skills can all guide me to an extent, but whether I'm hitting the right level for each of my students?
Bottom line - I'm not completely sure.