IMHO, it's okay to list a poster from a university conference on AMCAS. I definitely included mine. I think it's reasonable to have this listed on your CV, at least early on in your academic career. Once you get posters/abstracts/pubs in peer-reviewed journals, regional/national conferences, etc, then you should remove the university ones you had.
What makes me more nervous about the OP listing his/hers as a poster on AMCAS is that it's from a class and it doesn't sound like it's actual research. Sounds more like a final essay or something but in poster format, at least to me.
Just my $0.02.