Have you really sat down with the teacher -- when you are calm, not angry, and when you're able to be a bit more objective -- and asked her to explain what she means?
She may not mean "He won't learn his ABCs this year." She may be seeing maturity issues instead. And of course a kid can mature a lot in the coming preschool year, so those may all change too. But unless you really delve into the specifics with her, you won't know. Set up a meeting with her without your son there and at a time when you and she don't have other places to be -- so you have the time you need. Get specific examples of what she is talking about. Ask her why those specific examples demonstrate issues serious enough that she already believes he may not be on track for K in a full year's time. She may find, as she discusses this with you, that SHE is thinking differently about him as she mulls it over with your input. Give her a chance. From the post, it sounds as if there was no discussion, only a statement by her without any backing to it -- yet.
I know you're very angry right now, but can you cool down enough to ask, "OK, might there be things going on here that I need to know about?" and to be calm and non-judgmental with her enough that she can explain her thinking to you?
I agree that with a full school year still ahead of him, it's awfully early for her to make such judgements. Take your meeting with her as a time to tell her that you think so, and to ask that she and you get together regularly throughout the year -- not just at parent-teacher conference time-- to look at his progress together.
It may be the case that she is hedging her bets and fears that she may not be able to reach him for some reason, so she's lowering your expectations from the start. That would be more about her than about him, but does not necessarily mean she's a dreadful person or awful teacher; however, you would want to be sure she doesn't label and pigeonhole your son as "not going to K" and thus doesn't work with him as much as with kids she thinks are moving on.
Again, I'm not saying she's right, but do listen to her. Is she experienced? Or new or new-ish to teaching preschool? Does she have long experience with kids this age so that she might see things a parent doesn't? Or does she possibly tend to label kids early in the year? (Other parents of past students can fill you in, if you know some.)
Go back and clarify rather than being angry, and work with her wherever possible, noting of course that you expect your son to get her full attention.
As for maturity -- if, by the end of the year, he has all the ABC and 123s nailed but has maturity issues, he would indeed not be ready for K. Kinder requires kids to be ready to do things in a group, move from one activitiy to another without resistance or meltdown, share, listen to an adult who's not mom or dad -- and preschool helps them learn these things. He may be doing them already, which would be excellent! But those are the key things to watch for in terms of K readiness, not just counting and letters.