A Chedworth Avenue property in Bishopdale was broken into and burgled about 2pm.
The homeowner said the incident was absolutely devastating and the neighbours were quite shaken up.
She said offenders broke in through the back window and turned the place upside down.
Cotswold School, Isleworth School, Kidsfirst Isleworth and Breens Intermediate all went into lock down between 2pm and 3 pm, when police found out a firearm was involved.
A man has been arrested following the incident and police are still looking for three other possible offenders.