You raise an excellent question that isn't asked often enough! Successful instruction of any kind - whether it be obedience training for dogs or beginning calculus - begins with establishing appropriate learning outcomes/expectations. At one point in my career, I was responsible for conducting teacher evaluations - I used an approach based on a clinical supervision of instruction model. One aspect that I was especially interested in was whether or not the teacher had set realistic expectations for the class. I reached this somewhat subjective determination by assessing the alignment of the lesson to the students' present skills and/or knowledge. However, sometimes even when a lesson is aligned with students' abilities (i.e. appropriate expectations), things can still go awry if the teacher's methodology is inappropriate or poorly executed. It took me several years of experimentation to develop an effective method of teaching my most difficult special ed. students at the worst times of the day (30 minutes immediately after lunch and 30 minutes before dismissal). I learned that even at such times typically considered not to be conducive to instruction, with the "right" method, underachieving students easily surpassed my expectations by several grade levels within just a few months! So, realistic academic expectations should always be in synch with students' achievement levels and can be realized only if the teacher effectively employs an appropriate method of instruction.