This answer entirely depends on what is being sampled. Is it whether choir is superior to other classes or not? Most likely very biased. Is it about favorite range to sing at? Maybe a little bit biased. Is it something that has nothing to do with choir? Then most definitely not biased.
Again, it entirely depends on what is being asked. One cannot base just "bias opinion" on "20 students in choir" without knowing what the question being asked is.
