When did “&” stop being taught alongside the alphabet?

Uh, it never was? I didn't learn what the ampersand was until middle school.

I knew how to interpret the symbol (because relatives and teachers corrected me when I mistakenly read it as a capital letter "A" when looking at newspapers, as early as kindergarten, while trying to demonstrate my ability to read), but I was taught to avoid using it as part of handwriting. I knew what to call it, and that it means "and" but was encouraged to use the plus sign (+) if I needed to use punctuation for such purposes, although even that was frowned upon, as improper and informal.

During middle school, we were taught to type using computers, and so learned some of the more common symbols. While typing, shorthand was tolerated with greater sympathy, since printed or typed writing was clearly legible, and not subject to the harsher criticisms of penmanship, where abstract symbols might look like childish scribbles.

I didn't develop my ability to write an ampersand by hand until high school, when I was forced to take notes in class, and use of shorthand was acceptable, as long as I could read my own note taking, and recite a reasonable summary of classroom lectures if called upon. My notes never actually lived up to that criteria but at least I could write an ampersand.

More importantly as not discussed, the '&' character also derives also from the french word 'et' which refers to 'and' in English. Something we can call a Norman influence.