I hate, hate, hate when schools assume that parents/grandparents can and will attend. It is discrimination & just plain mean towards the kids.
It doesn't even matter if they gave you notice (though lack of it makes it even worse).
I had started to have this 'discussion' with dd's school before we moved to the US, where things like this are far more flexible and inclusive. (both from pov of employers letting parents take a couple of hours off, and the schools making sure kids don't miss out).
I would gladly go into battle for you on this! But I feel a random stranger writing form the US might not have any effect.
The school can usually cope with x number of pupils. Just because there's a different activity going on, doesn't mean they can't cope without 100% parental attendance. The teacher could and should have taken those 3 kids in with them. They're not allowed to discriminate if parents can't afford to pay cash for an activity, why do they think it's OK to discriminate cos parents can't always afford to take time off work (which often costs money)?
And don't get me started on how this is just one more way that women are brow-beaten to fit into the patriarchy.
Your child has been singled out, left out & punished because of the school discriminating. How you react to that is your choice, but I was ready to go to governors/LEA over this issue until I learnt we were moving. Why don't they decide that all kids whose parents have brown eyes have to sit in a different corner? What about if the parents have brown skin, do they pick on those kids? Or kids whose parents are disabled, maybe they should be laughed at in assembly? No other form of discrimination is even close to being acceptable, so why do they think that this is?
btw, I'm sure that there should be laws about discrimination... oh, wait, yes, there are.