It’s an interesting one kitty. I’ve not heard of it before, certainly depending on the timing it could be awkward for anyone needing to go straight from work whether work requires a suit or very casual clothing. One of our sons works shifts over a 7 day period and so could well need to attend a school event without the opportunity to return home and change. Has anyone asked the school for their reasoning? Maybe an attempt to make it ‘special’ for the children celebrating their achievements? Smart casual can mean many things and I suspect will trigger a wide range of clothing and no, I doubt any parent will be refused entry. Many congratulations to your grandson, you must all be very proud of him.