\(\displaystyle \frac9{10}=90\%\), so the percentage of this sample is calculated correctly.

Ten volunteers provide limited evidence about the whole school. Maths-club volunteers are also likely to have a different interest in maths from other pupils.

A larger random sample from across the school, using the same neutral question, would provide more representative evidence.

Answer

The sample is small and maths-club volunteers may not represent the school. Use a larger random sample across year groups.