Although it may sound like some sort of wizard spell, elhi is actually a combined abbreviation of elementary school and high school, minus the "school" part.
Elhi is used in educational contexts to refer to activities, curricula, and other things relevant to or appropriate for students in all grades of primary and secondary education. In the U.S., that's grades 1-12, covering elementary, middle, and high school. (K-12 is a similar expression except that it also incorporates kindergarten, which is typically not considered elementary school.) You might see elhi used to describe schools, textbook publishers, educational apps, and more.