While the timing of the event might not be a proper time (for you), I completely disagree with the notion that it is a lack of menchlichkeit & a lack of seichel on the school’s part. Maybe the school actually used some thought and that was the best time to make it. I would even say to not be so selfish that since its not going to work for you, that you just blame the school for its lack of menchlichkeit and seichel. Why not just call the school/teacher and see why the event is held when it is before jumping to a conclusion like the above. Nowadays, it almost seems easier to just go on a website and complain about something without even taking the time to see what the full story is.
Should we say mothers of kids are lacking seichel as well when they make a party in school for their kids and the fathers can’t come because they are working during the day? Or maybe its a lack when the school makes shacris for the kids at 7:30 when its better for me (as a father) at 7:15 so I can daven with my child.
Also, is it not worth a few unpaid hours to all mothers (and grandmothers) to see some nachas of your kids (gandkids).