A is the alpha of the alphabet, foremost of the vowels. It provides an excellent introduction to the pitfalls of phonics teaching. Like all vowels it can be "short", as in cat, or "long" in the digraphs ay and ai. Long a may also be invoked by the presence of another vowel next-door-but-one, especially e, i or y (the part-time vowel) as in name, basic, and lady.
So far so good, but English vowels like to show off their infinite variety. The letter a also contributes to aw sounds (as in law, maul and all) and ar sounds (as in ah!, car and father). In some words, like grass and craft, it gives a different sound depending which part of the country you're in. Paired up with other vowels it can create more sounds. The ea combination is particularly versatile: think of read, bread, ear, wear, heard and heart.
And a also does service as the most common English sound, known as the "schwa" - that sort of half-hearted grunting noise in words like Russia, cellar, postal and adopt.
Still, while it's as well to be aware of such pitfalls, we should teach them with discretion.
The National Literacy Project sensibly suggests introducing "short a" along with the other letter-sounds, then - over the rest of the infant years - adding "long a", other regular vowel digraphs, and odds and ends like all (best covered in rhyming groups, for example, all, call, ball.) More esoteric sound-values, including the notorious "schwa", should be mopped up during spelling lessons in the juniors.