Sorry, I am not buying the “Summer Heat” excuse. This is Summer in England, not Tucson.
When I went through grade school and high school boys wore dress pants or jeans, your choice. Girls had choice of dress pants, jeans or dress. NO Shorts in school but permitted for after school practice of school sports and we had no AC in the humid and stifling heat of eastern Kentucky.
Heck I never had AC at home until I was nearly 11 yrs. old! You used a window fan and floor fan. You stayed outside until dark, then clean up and bed time. Pansies need to suck it up buttercups!
People blood get accustomed to certain temperatures in the area that they live in. If you took Tucson students and put them in England during the summer when it is hot they probably wouldn't think it was hot but some would probably would consider England cold from what they are use to.
In the UK hardly anyone has air conditioners and even fans, it is a great place for American to visit during the summer.
I'm pretty sure those schools in the UK with out air condition or fans get pretty hot during summer.