Because if you have a school uniform, it is considered to be the most formal piece of clothing that you own, in much the same way that a military dress uniform would be considered appropriate for formal occasions. read more
However, unlike funerals, you do have time to prepare, and there is also a decent market for renting formal dresses and tuxes for kids (which I learned about recently), so parents might choose to let their kids dress up for such an occasion. read more