I am 70% sure CHS is a 7 to 12 school as a great number of high-schools are nowadays meaning the Wondercolts Forever: The Diary of Celestia and Luna chapter book about Luna's freshmen year and Celestia's senior year would make them 6 years apart which would support this cuteness. XD

(Its the only way the CMC going to CHS makes sense.)(Oddly the book says Luna came from middle-school so who can say, but hey the books are third tier cannon.)