Schools honestly cannot win - 30 small people and 1/2/3 adults in one room makes for a ridiculously hot environment. Open windows and doors do little to help if there isn't much of an actual breeze outside.
Take them to the park?? Are you joking?! We have to have appropriate ratios to take them off site, and that's unlikely to be made up of just school staff even if you took every child and every adult.
Schools aren't set up for the high temperatures we are seeing now - we have them so rarely that they've never needed to be. Would you rather your child came home with a headache/feeling faint or dizzy/sweating profusely just by being in the classroom? What if they were sick in the day due to the heat and you were called to collect them - would you think they should have just shut the school to start with?
This isn't aimed at the OP particularly, just in general really.
Schools don't take the decision to close lightly, the same as they don't - shock horror - take the decision to close for cold weather or snow lightly either.
There is a minimum temperature that must be reached for schools to open - if a boiler breaks in the winter, unless alternate heat sources can be found sharpish then schools can't open that day. If staff can't get to school because of snow - often not actually in the square mile surrounding the school - how do they teach? If the approach to the school site is dangerous - icy roads, being on a hill etc. - how do you make that safe for everyone? Because make no mistake, if a school opens in snow/ice/very cold weather, and somebody's little poppet has an accident on the way, you can be damn sure the school will get it full throttle from the parents for being open.
You can't really make the comparison to US schools - for a start, lot of US schools are already on their summer holiday, so they won't be at school for the hottest months of the year. They also have plenty of 'late starts' in the winter (in 'cold' states) to allow for bad weather easing slightly. Schools in the hottest states are built for the hot weather, same as in the coldest states people have transport/equipment built for that too. They have more 'extreme' weather so are prepared for it - we, as a rule, don't, as we don't expect it.
If schools choose to close tomorrow, it will have been a well thought out decision, with every possible contingency considered beforehand. Schools aren't doing this to make your life a misery, they're doing it for the welfare of their staff and children.