Here's why:
* The schwa sound (ə) is a very common, unstressed vowel sound in English. It's often found in unstressed syllables.
* In the word "pupil," the first syllable ("pu") is stressed, while the second syllable ("pil") is unstressed.
* The "i" in the second syllable is pronounced with the schwa sound.
Therefore, the word "pupil" is pronounced with a schwa sound in the second syllable.