Smoke can be seen billowing from the scene at the Heights on Hill Road, Haslemere, Surrey.
A video posted by BBC Radio Surrey shows the former school in tatters following the blaze.
Witnesses report that fire engines are on the scene.
The location used to be the site of Haslemere Prep School.
Twitter user @HaslemereNorman, present at the scene, said: “Hard to tell how bad it is.
“The smoke was so acrid I had to get back indoors pretty quickly. Thank goodness it’s a still day.
“Two fire engines were dealing with the fire but just heard a third arriving.”