The voice is not only the original instrument, but also the only way a teacher can see into the mind to judge a student's understanding. Recorders are used a little in earlier grades, but only along with singing. The aim of the recorder player is to imitate as closely as possible all the capabilities of the human voice. Students are Prepared for a new element through a repertoire of songs and games containing the unknown element. During Presentation the teacher uses a song that isolates that element, identifies it, and names it.
Practice involves using the new element in new repertoire.
The same approach works for instruments, but there are differences. The teacher can play a tune with the new note, and ask them to figure out what you are doing. These are new, challenging skills, but if students have good musical training and a repertoire of songs developed from kindergarten through third grade, they are ready. Yes, I'm strongly suggesting that the best preparation for instrumental studies is a strong vocal general music program in the lower elementary grades. This is exactly where American students fall behind their European cousins, and most never catch up.