For me I do not think that teacher's expectancy can influence students' academic performance in school subjects because every teacher is expected to provide an objective judgment on the course of carrying out the assessment of their students.
There seem to be a good support in literature that teachers expectancy positively affects students' outcome or achievement. You may to the works of Jeter, 1973;Cotton, 1989, Friedrich, 2015. I think the key thing is the effective communication of the teachers expectancy in way of positive mentoring, encouragement, follow up activities, support, etc.
In my knowledge YES please, very much. The higher communicated and defined teachers' expectations over their students' academicaly or/and socially the higher the students' motivation to learning and setting of higher goals, and vise versa. There are numbers of researches results which have come up with evidences. you can google nr. of them. solution is within the teaching techniques we learn from teachers colleges. Be knowledgable, tolerant and positive to your students, show them that you believe on them and their efforts, you have higher expectations on their future academic life and support, coach and guide them to meet their expectations. Good luck.
Yes, it is likely that expectancy influences - but variance and in what context- i.e formal and informal and/or math or English - needs to be conceptualized