"My husband and I are separating soon and I know the arguments are having an effect on her."
I was reading the posting, figuring it was a classic adjusting-to-school issue, when this sentence just appeared out of the blue. It should be the very first sentence of the posting because it's a vital clue to her stress.
Yes, she may be stressing about school itself, but isn't it extremely likely that she's stressing because she's in a new environment at a time when her home environment is not just changing but totally turning upside down? Even if you haven't told her yet about separating, she IS much more aware of tension than you and your husband realize.
Does the school counselor know that you are about to separate and that there is tension in the home and she is aware of that tension? Does her teacher know this? I think both of them need to know immediately. That helps them deal with your child so much better -- from a position of being fully informed. You do not have to give them tons of details or get too personal. But you do have to let them know that your little girl, who has never had the experience of going away from you to any school-like environment (because she didn't do preschool), is now in her first time away from you while simultaneously dealing with tension at home.
The teacher and counselor should work with you to make her more comfortable in school. She will need school as a safe haven where she feels good during the time when she fully realizes daddy is not living at home.
You cannot alter the way the school is run. Instead, step up and at least pretend to be enthusiastic even if you are not. Have her measure an apple and an orange and something else around the house with YOU, and be very upbeat and excited about it. Have her "help" you write something like a shopping list (just two or three words of one, using only letters she knows how to form now) and praise her like crazy and then above all, connect it to how wonderful it is that she learned that IN SCHOOL. Ask the teacher for other ways to reinforce not so much the learning as the enthusiasm for whatever it is they are doing -- even if you do not just love it yourself. This is for her, not for you. She is having to deal with her first separation from you at an age when, unfortunately, she must start right in with academics -- that is just the way school is now; in kindergarten kids are doing things that once were done in first grade, etc. You can't change that. But you can make her think that mom thinks she is doing the coolest stuff ever.
Meanwhile, you really, really need to enlist the help of the teacher and the counselor. The counselor may be less apt to cancel those talks once you tell her or him about the real situation at home and once you make her aware that your child did not go to a preschool and is having her first separation from you.
LIke another person posted, consider getting your daughter into counseling now so she can start to prepare for the separation. And yes, sending her to preschool (fewer hours a week) and starting K in another year could be an option but you need to decide that soon.