Helen Mann, 43, had became head of Sytchampton Endowed First School near Stourport-on-Severn, Worcestershire, just six months ago.
But she is thought to have taken her own life shortly after holding a teacher training day. Paramedics were called after the alarm was raised by a member of staff but the married mum-of-two was pronounced dead at the scene.
It is understood that no children were in the building at the time.
According to the Sun, Mrs Mann's devastated father-in-law Kenneth Mann yesterday said he had 'no idea' why Helen - who had two sons, George, 13, and Jack, 10 - may have wanted to commit suicide.
He said: "We don't know the exact circumstances but we are all deeply shocked and absolutely heartbroken.
"Helen only started in the summer. She was the loveliest person, so nice to everyone. They have two wonderful sons who she loved so much.
"It's come out of the blue, we never expected anything like this."
Police have interviewed Mrs Mann's husband Philip, 48, and a report is being compiled for the coroner.
A police spokesman said Mrs Mann's death is being treated as "unexplained but not suspicious".
When she was appointed, Mrs Mann, of Worcester, spoke of her excitement upon joining the school, which has 109 pupils and had achieved two 'outstanding' Ofsted reports.