Teachers are worried about a dangerous material called asbestos that used to be used a lot in school buildings.
Asbestos used to be very popular with builders, but if it gets damaged, small particles are released into the air which are harmful to breathe in.
An investigation in Sussex and Kent found that nine in every 10 state schools had asbestos in them somewhere.
The people in charge of health and safety say it's safer to leave the asbestos where it is than to remove it.
Asbestos under a microscope
They say that if the asbestos is in schools and sealed in place then it represents "no health risk" to those using the building.
But teachers' groups want all asbestos removed from schools. That would mean closing the school buildings while the work was done.