-what is the purpose of studying "English" / "Language Arts" / "Literature" in junior and senior high?

Critical thinking. Language is what civilizes us and a study of language is a study of the basis of civilization. By building familiarity with the fundamental mechanics of communication, you are aiding your students in building a familiarity with the fundamental mechanics of culture, rhetoric, interpersonal relations and what it means to be a member of society.

-what are the most important skills that a grade 12 student in 2021 will need to possess? (Think vertical: something that can be implemented in grade 7 and refined until the end of grade 12)

Critical thinking and analysis. There is no shortage of written material right now and the amount of blogspam and useless persuasive pieces will only increase. Your task is to give your students the tools to evaluate every piece of communication they encounter for truth, accuracy, lyricism, bias and effect.

-should reading be divorced from writing? The focus of formal writing assignments in upper senior high are literary essays: one text, thesis statement, focused paragraphs that explicate the text in the thesis' direction, a conclusion that wraps up the argument and gives the "so what?" of the entire paper. Is it necessary in our world today to have such a complex and highbrow assignment, or should reading and writing be taught and assessed using discrete methods?

Writing and the construction of an argument is essentially the only place students study rhetoric anymore, and rhetoric is the art of convincing someone (or being convinced). Unless you wish your students to be gullible, credulous slackjaws that believe everything Jeb Bush says, they need the experience of shaping an argument themselves to see how it is done.