About 150 pupils from Colinton primary school in Edinburgh were moved at around 11am after a gas leak was discovered. They were taken to nearby Oxgangs primary.
Their parents were told to collect them as usual at lunchtime, which is when the school day ends in the city on a Friday.
Meanwhile, a school on Lewis was evacuated after a chemical spill in a science lab.
Fire crews were called to the Nicolson Institute in Stornoway at about 10:15am.
The school has around 1,100 pupils, who were evacuated. Western Isles Council said that it was done “as a precaution”.
It is thought the chemical involved was ammonia.
Highland and Islands Fire and Rescue said that three fire engines were at the scene.