There is a tendency on Wikipedia for editors, either consciously or subconsciously, to try and base inclusion standards so that they don't allow too many articles on a particular kind of subject. I call this the "Pluto principle". Pluto was considered a planet for 76 years until it was discovered that there may be a shitload of "plutos" in the Kuiper belt. Well we can't have too many "planets" in our solar system or it would be impossible to teach school kids to memorize their names so the requirements were tightened. The same thing seems to be happening on Wikipedia. We presume that high schools are probably notable because it's likely that they have had some coverage in local media even if finding that coverage online might be difficult or impossible. However, the same presumption can be made for primary schools and perhaps any business of significant size and longevity but there are a lot more businesses and primary schools then there are high schools but giving them the same "benefit of the doubt" might lead to too many articles on such subjects.