This is one of those "meta" tags.
Currently there are 31 questions with this tag: https://math.stackexchange.com/questions/tagged/high-school
There are 4-5 questions which specifically talk about high school, but I believe they can do without that tag.
If we keep the tag around, I predict many questions will get this tag just because the asker is a high-school student and will be practically useless (if there was any use of it in the first place...).