Read the following experiment:
A group of people are directed to watch two teams playing basketball. One team wears black, one team wears white. The group are asked to count the number of passes made by Team White. The action is fast-paced and non-trivial to follow. Halfway through the video, a woman dressed as a (black, for obvious reasons!) gorilla comes into the middle of the court, beats her chest and leaves. She is on the screen for 9 seconds.
Approximately 50% of the group will not see her, and indeed when they do notice on a repeat view will strongly believe that she wasn’t there on the first viewing.
This shows a lot of things, but crucially that we only have a limited budget of attention. You shouldn’t try to work out the square root of 5663 while circumnavigating the Champs-Élysées roundabout for the same reason.
Likewise, we should reduce the emphasis on exam success in our schools, as there is more than an outside chance that all this focus on a C at GCSE might well mean a teacher is forgetting to do something more important, namely educating our children.