Imagine going into school when you're feeling exhausted, weak or even in pain.
That's the reality for loads of children with a disease called sickle cell, and doctors are complaining lots of teachers don't do enough to help them.
People with sickle cell have slightly different shaped blood cells to the rest of us.
Because their cells are curved they can't squeeze through blood vessels so easily, which makes it harder for the blood to move through the body.
That can cause loads of problems, like leaving kids really tired or in a lot of pain.
Lazy vs tired
Now a big study has found lots of schools aren't understanding enough about the condition.
Professor Simon Dyson, from De Montfort University, who was involved in the research, said: "The teachers may know that the child has sickle cell disease, but they really don't appreciate the full range of factors that can influence the child.
"For example, they may regard the child as lazy when in fact the child is simply tired from their sickle cell disease."
It may not be something we hear a lot about but 15,000 people in the UK have sickle cell and around the world children are born every day with the disease.
So experts say it's important we all try to find out a bit more about it.