Answer: Yes, the poll is biased.
Step-by-step explanation: The poll was biased when it was performed because it was conducted with only students who were attending a basketball game. Since they are attending a basketball game there is a very good chance that they are biased in favor of basketball.
A better method of determining which sport is more popular would be to conduct a poll in a neutral area. The poll would be neutral if conducted in a lunch room or some other non-sport area of the school.