No, go and enjoy the class. I think you will like it. Trust me, the whole class is watching the teacher, not you
-- However, I do tend to sit in the back, behind everyone, to avoid my false perceptions of being watched. I also realized something kind of cool the other day -- my skinny friend and I went to an aerobics class and I could do more than she could -- who would have ever thought!!
Being thin doesn't automatically mean you are in shape.