Are questions about middle and high school science fairs on-topic? Of course, they wouldn't be about the content, they would be about the process of doing the research in the fair.
It will strongly depend on the particulars of the question.
- Questions related to science fairs per se will generally be off-topic as being undergraduate or below.
- Questions related to the specific topics of research will typically be off-topic on this site but on-topic on another StackExchange site (e.g., Biology.SE).
- Questions about general research process and presentation, whether to mention your success in a grad school application, etc., would generally be on-topic here.