More than a million children in the UK have asthma, and many of them are able to keep it under control using inhalers.
But Asthma UK's study found that in some cases schools have stopped kids from carrying around their inhalers.
Instead, the school looks after them to keep them safe, and the children can use them when they need to.
Doctors say sufferers should always have them nearby.
The study, to mark World Asthma Day, found that half of the children asked said they had problems joining in with lessons and going on trips.
The charity wants a number of improvements:
• More school nurses
• Better guidelines in schools for helping kids with asthma
• All schools should have an asthma policy in place
A spokesperson for the charity said: "There is no need to wrap children up in cotton wool but it is every school's responsibility to ensure that teachers have the knowledge and resources to protect and support the children in their care."