In a non-confrontational way, you might raise that there seem to be some coordination and communications issues lately, and you want to make sure you both are on the same page to avoid conflicts and confusion.
The kids are probably struggling with the transitions. Once that takes place, they settle down. Very common. There may be factors not related to either of you in their lives as well. A tough day at school, not enough sleep, missing a friend in the neighborhood of your W's place, and so on. Whatever you do, know that this isn't personal. They don't have the language or understanding to fully voice their struggles, and so they voice it in a way they do know. When you get a chance play one-on-one with them at their level (silly, child-like play). They'll be more likely to open up and allow you to help process their feelings.
On W's butting in, I'd say that you are worried about the kids being caught in the middle and doing what's best for them. Routine is best, as it gives them stability and boundaries. So, unless there is a special occasion, the person who doesn't have the kids that week doesn't ask or offer. It is up to the host parent to ask for help if needed. Also, the expectation to cut down on stress is that the children are OK unless informed otherwise. You respect her as a responsible mom, and you want the same in return.
As far as the school, let it drop for know, as I'm sure she's got some rationalization defense & will react poorly to a challenge. She'll either tire of contriving excuses to show up where you are expected or not. Deal with it if it continues as a pattern, but shrug this one off. There could be many reasons why she might be doing this. Control seems to be a big deal between the two of you, and I suspect she is not feeling all that much in control of her life right now. But, as I said, could be any number of things.
Hang in there, and hope things improve.
Me: 50 W:43 S6, S3 M: 12 yrs. T: 17 M is bad & Not happy Bomb Mar '14 S 5 Feb '15 D Bomb 13 Apr '15 (but "no hurry") DB Coach May '15 Wants proceed on D Aug '15 Starting 1-on-1 negotiations Sept '15