I have discovered that people who are approximately five years or more younger than I am weren't taught all of the vowels, and that even people my age and older don't always remember the missing vowel.
The vowels are: a, e, i, o, u, and sometimes y and w.
W is to vowels what Roebuck used to be to Sears & Roebuck or what Western used to be to Country & Western. It's a forgotten member of the team, important once, but kicked to the curb over time.
My uncle (my stepfather's brother) used to win bar bets because he knew what may be the only word in the English language that uses w as a vowel: crwth. (A crwth is a musical instrument.)
On behalf of vowel lovers everywhere I say: Teach W!
But maybe we should tell children that the vowels are: a, e, i, o, u, and sometimes y and on extremely rare occasion w.