In the teaching of reading, a schwa is generally thought of as a vowel sound in an unstressed syllable that does not make a discernible “other” vowel sound. The “a” in “about,” for example, which does not make the vowel sound in “cat” or “make,” or to use an example from the comic, the second vowel sound in “onion.”
(Note that not all unstressed vowels are schwas: the second vowel in “pillow” is not a schwa.)
What the comic does here is to combine, or maybe conflate, the short /u/ sound with the schwa. Short /u/ is the vowel sound in bus, chump, and oh yes, stuck, truck, Doug, and ton (even though the last two are not spelled with a u to symbolize the sound). This makes some sense: if I had to correlate the schwa of “around” with an existing short vowel sound, I’d choose short /u/, and I think most people would not distinguish the /u/ in bug from the first sound of about.
But–in the phonics of reading instruction anyway–there is a difference in concept if not always in sound between the schwa and the short /u/, which typically appears in a stressed syllable. Most of the examples in the comic are short /u/ words, not schwas.
You’re right that many schwas can be represented by other letters and they’d still be pronounced the same way. I taught primary grades for many years, and it was always fun to see what kids did when they were spelling words with schwas on their own. Take a word like “wagon,” which would come out “wagun,” or “wagin,” or “wagen,” in addition to “wagon” if we were lucky and conceivably “wagan.” Your pencil example is another good one.
Disclaimers: This is how reading teachers are trained to think about schwas. Phonologists and phoneticians may have a different way of approaching them. Also, I haven’t taught reading in about 25 years, though I have written reading curriculum materials. It’s conceivable that things may be changing.
