High School students need help too and I don't understand why only higher education matters. Why can't academia support the questions of high school students too?
TL;DR: It's not that high school questions are unimportant. But they target a different group of experts than questions about higher education, so they need a different site.
The Stack Exchange model is based on the idea that a high-quality Q&A site should have a dedicated user base of experts in that subject, who spend time contributing high-quality answers to the site.
The group of "experts on academia and higher education" and "experts on high school" are distinct groups with not much overlap. So it makes sense to have separate SE sites for higher education and high school - the expert user base will be different. Academia is the site for higher education. There is currently no SE site about high school, but you can check Area 51 to see if there's a proposed site, and propose one if there isn't.