That is how the real world works. Your boss doesnt have to accept your reason for being late to work any more than the school has to accept your kids reason for being late to class. Its not a power trip, its an understanding of expectations.
Blowing up at the school for expecting your kid to be on time set a bad example for your kid, as it gives them the impression that they should be able to expect someone else to accept thier reasons for things, and we all know that just isnt true.
As for the dress code itself.....I dont know where you do your shopping, but I would assume that if someone is comfortable enough to go to do thier shopping in pajama pants, slippers and a hoodie, its quite likely that those same people would feel comfortable going to the kids open house at school in the same thing.
People Of Walmart - Funny Pictures of People Shopping at Walmart : People Of Walmart is a real website. I wont link to it directly, but if you are interested, go check it out. Ill just preface it by saying that I believe in my heart of hearts that the vast majority of people you will see there would indeed wear that **** to the kids school to handle business.