Mary, I am also curious about the "invoking spirits of ancestors" being defined as a religious activity...how did the instructor conduct this exercise? Was it a written assignment? You mentioned "chanting"--is that what this was referring to?
Everyone has ancestors--and writing instructors frequently ask students to write about their past or their families or a traumatic event in their lives or their ethnic heritage--so without more information the context of this is hard to gauge. I'm not sure what is "religious" about this but if the teacher is in fact seeking to engage students in some sort of ritual not pertinent to the course description, and if this is making students uncomfortable, then this is a problem. But sometimes these sorts of activities are used in classes where dymnamics are being explored--like acting, or communications. I occasionally employ somewhat experimental methods in my creative writing classes...fortunately I am in little danger of offending anyone's religous sensibilities because of the campus climate where I am currently employed--but I have been called a "godless heathen" by at least one student at another school before....

I think before we can help we need more information.