Shropshire Fire and Rescue Service crews from Bishop's Castle and Clun stations were called to a fire at St George's CE Academy in Clun at around 6.40am on Tuesday.
Headteacher Rebecca Manning said: "Early this morning, a couple of hours before children would have attended the school, the cleaning team discovered a small electrical fire in the staff room.
"The issue was dealt with extremely quickly by the fire service, who were then helpful in ensuring that the site was as safe as possible.
"We are grateful to all those who helped, and of course, for the fact that everyone is safe and sound.
"The required checks have been made, and the school is safe. However, there are various areas that need to be thoroughly cleaned due to the smoke from the fire, and the school will remain closed tomorrow, Wednesday, December 15. We fully anticipate that the school will reopen on Thursday, December 16."
Two fire appliances were mobilised, and operations and fire investigation officers were in attendance.
The crews were equipped with breathing apparatus, both a covering jet, two hosereel jets, a thermal camera and positive pressure ventilation to make the area safe.
The log states that there were reports of a microwave fire in ground floor staff room.