We recommend introducing schwa in second grade as it relates to spelling because that is when it becomes most important and needed as students are writing longer words. However, all teachers should understand how the schwa sound functions to help explain to students why a letter is making an unexpected sound.
For example, consider the word about. Kindergarten and first-grade students might be writing all about me or all about a particular animal. They often will spell about with the letter 'u' at the beginning. Even though younger students are not ready to learn about schwa, teachers can explain that the vowel is making an unexpected sound there and that we call that sound schwa, letting students know they will learn more about that later. English is a language full of patterns and while schwa might feel like it is irregular, we can teach students when to anticipate it in words and prevent the impression that English is an irregular language.
You may introduce reading words with schwa later in first grade or early second grade. As students are reading words like basket and problem, you can explain what is happening in our mouth, teach the "Call the Dog" strategy, and model how we flex the vowel to read a word that we know.