It's a valid concern.
Most teachers are GREAT. Awesome professionals who work their butts off, and really get kids, and really understand that some asynchronistic development is common with most kids (and major asynchronistic development will be true with at least 1 or 2 kids in their class every year).
And sometimes you get the ones you have to wave a neon sign in front of them and they STILL don't see the nose on your face.
Most teachers like/respond well to questions. As in; "Is it a concern that his writing is so far behind his reading level?" or "Will his writing hold him back, since his reading level is so far ahead of his writing level?"
Either will both get across the concept that her reading is advanced of her writing... but it also starts a dialogue/conversation between you and the instructor where you can talk about your concerns without it being a "that mom" kind of thing.
A good teacher will run with you, and work with you throughout the year in a friendly/open way. Since most teachers are good, you have a better than even chance that it won't be an issue at all. A bad teacher, well... there are whole BOOKS written on advocating for your child and the best and worst ways to go about doing it... but why put the cart before the horse?
But as for experiences like that? Absolutely. My son is technically either gifted or 2e depending on how you look at it (2e = gifted + either a learning disability or disorder) and asynchronistic development is a common facet of gifted kids. As an example; he was reading at a 4th grade level when he was 3 (btw, he's now in the 4th grade and still reads at a 4th grade level, because he got bored by reading, so didn't take it any further), and doing algebra at 6 or 7 (math is continuing to skyrocket). But to say he has a doctor's handwriting is being kind. His handwriting is atrocious. His 2d spatial awareness is ALMOST NONEXISTANT. (Mechanical writing, puzzles, tangrams), but his 3d spatial awareness is mindblowing (he can't do a 10 piece puzzle in under an hour, but he can do a 30 piece 3d tower in under a minute. He just reaches out almost randomly and stacks them). He does algebra and precalc with ease IF they are equation based, but a simple Susie has 3 apples and Johnny has 4 pears word problem has him staring at the paper blinking for 10 minutes. And then he's nearly always wrong.
For my son some things just come EASY to him, and some things he struggles mightily with. Because his development is so asynchronistic, we actually are spanning 7 grade levels in his work. So that the areas he excels in he can continue to pursue at a level that is fun and challenging for him (aka appropriate), and the areas he struggles in we can move slowly with so he doesn't get so frustrated he just gives up (aka appropriate). It's quite challenging from a teaching perspective.
Which is why students in a classroom setting are (ideally) broken into ability level groups. For elementary, reading groups are the most common breakdown to come across. (Later on in middle school more breakdowns occur; math, english, science, history... in highschool ability level classes are broken down further... and in college they're broken down completely into ability level classes. Not just 100-400 level classes, but 101, 127, 192... 200, 240, 283....304, etc.)
The first several weeks of K are going to be the teacher evaluating where each of her kids is at and trying to group them appropriately. Not just from an academic standpoint, but also an emotional and social one. AKA she won't be able to tell you a lot those first few weeks. Then there will be about a month of "solid" work, and then parent teacher conferences happen. The same is true of every grade, but K requires the most "sift and sort" work for the teacher. Because there will be kids who don't know their alphabet (by sight or verbally) and kids like your daughter reading at the 2nd grade level.
It's going to take a bit for things to get into a groove, but as long as you've got a good teacher (and that most are); you'll be able to start that dialogue/partnership... you'll BOTH be fine.