"There is a best singer in my class." only means that there exists a person who sings better than everyone else. It could be that everyone in the class sings horribly, but there is always a person who is the best of the worst. The kid who is merely very bad is the best of the horrible singers.
I think that you may be trying to say "There is a good singer in my class." - There is a person in my class who sings well.