I ended up responding with something short and neutral: "If you'd like to be included in calls in the future, I'm willing to coordinate. Would you like to know the details of what I discussed with his teacher?" So far, no response.
Just about everyone I talked to said I didn't do anything wrong here. I checked our mediation agreement - it says we each have the right to consult with teachers.
One big takeaway is that XW is still trying to control me, and is apparently more interested in her image than in the actual contents of the convo with the teacher.
Me-47,XW-43 S13,S16 M:18 BD:4-23-17 W filed:7-17-17 (5 months of in-house separation hell) W moved out:1-6-18 D granted:2-15-18 Decree signed:3-29-18