Humans use 100% of their brains all of the time. For a while there was a myth that we only use 10% of our brains but it’s been shown to be wrong.
When brain specialists (they’re called neuroscientists) have run scans on people’s brains, there is activity in all areas and it doesn’t matter what the patient is doing at the time. All the areas of the brain work together to carry out the most basic functions. It explains why if someone has even a small amount of brain damage, there will be some tasks that they can’t do as well as they could before.
It means that even if you’re sitting in class daydreaming, you’re probably using all of your brain!