At Gilmour Juniors we use whistles and bells which, if you aren't used to or expecting them, can be quite startling!
We regularly use a whistle:
When we blow the whistle it is a signal for all children to stop what they are doing and stand silently, the teacher on duty will then ask classes to walk to their lines.
The school bell is more difficult to hear if you are not inside the school building but is used at:
The fire alarm has a different sound to the school bell.