A whole school assembly, usually delivered by a Headteacher, can be used for the situation involving the death of a pupil or a member of staff.
The benefit is that the entire school receives the same news at the same time, keeping speculation and rumour at bay.
Follow the assembly with some classroom time for pupils to express any thoughts or feelings around the news that they have just been given.
End with something practical to do such as writing cards.
You may find our Saying Goodbye section helpful.