YANBU @artline200 ... Some schools, for some inexplicable reason, seem to assume that ALL mothers are stay-at-home-mums OR work very part time, like 10 hours a week, and can just switch their hours about willy-nilly to suit THEM and their 'school events.'
When my 2 DC were little, I worked 3 days one week and 4 the next (30 hours a week,)- in a semi professional role which was quite demanding with a fair amount of responsibility... Including interviewing people/seeing people who had appointments, attending court hearings and solicitors with people etc...
The amount of times the school/teachers/head etc, expected me to just drop hours and days, and switch days around, and basically move my life around to accommodate the fucking SCHOOL was phenomenal! I was fuming over one thing or another every other bastard week. Hilariously, the school never expected my husband - DCs FATHER - to upend HIS life for their convenience! Just me, the MOTHER! 
I even had ONE teacher - female, 30s, no children herself - suggest maybe I shouldn't be at work if I'm not able to give 'my all' to my kids when they're at school. I was livid. Complained to the Head, but I don't think he honestly gave a shit. Probably agreed actually. These same people will look down their nose at stay-at-home-mums for being 'freeloaders' though. As a mother of young children, you simply cannot win!
When my 2 kids (1 year apart in age) were 8 and 9, I changed jobs, dropped to 2 days a week, (16 hours,) and had much less responsibility. I did that for ME as my other job was so stressful, but it was much better being able to give more to my kids. Working full time - or almost full time, 30 hours a week like I did - was just too much. I dropped the hours and responsibility for my kids and ME though, not the fucking school, or because they thought I should!
But yeah YANBU. Schools are really unreasonable and demanding. And yes, they assume ALL mums don't work OR work less than 10 hours a week in a job where they are able to swap their hours about at the drop of a hat, and they have fuck-all responsibilities in their job. (And also no other commitments, like other children or other family members to care for!)