Never judge a book by its cover is a familiar proverb. I was reminded of this when Susan Boyle appeared on Britain's Got Talent. An unmarried, 47-year-old Scottish woman emerged on to the stage to talk to Simon Cowell before she sang "I Dreamed a Dream" from Les Miserables.
She was not wearing fashionable clothes or sporting a modern hairstyle. As we saw her for the first time I am sure we all began to make assumptions, so I decided to tackle this in an assembly.
The crowd sniggered as Mr Cowell asked her why her ambition to be a professional singer had not worked out so far. Miss Boyle was later reported in The Sunday Times as saying: "I knew what they were thinking and I knew that they were laughing at me. But I thought, `well, they'll soon shut up when they hear me sing.' And they did."
You could then play one of the YouTube recordings of her audition, which has been downloaded more than 100 million times.
Ask pupils: "When you meet someone for the first time, how do you judge them? How long does it take before you form an accurate impression? On what do you base it? Can you tell whether a person is kind, fun or selfish, just by looking at them?"
Get them to consider how their impressions changed after they heard Miss Boyle sing. The judges' impressions changed instantly and Piers Morgan gave her the biggest "yes" he had given anybody.
Point out that we make a lot of judgments based on appearances. Ask pupils to think about the last person they met for the first time. How did they react? Like Miss Boyle, did they read their reactions? Why do we usually look only at the cover?
Some researchers believe that this is because when early man came across another human or a wild animal, they had to make an immediate decision about whether they were safe. We have kept this instinct. But, as our brains receive more information about new people, we can change our minds on the basis of evidence.
Look at how pupils and staff form stereotyped opinions of groups at school (the emos, goths, geeks etc). You could also explore the New Testament story of Zacchaeus the tax collector (Luke, chapter 19) visited by Jesus, to the crowd's surprise.
Like the judges on Britain's Got Talent, we should always beware of judging on first impressions. Try to get to know new people and find out what's inside.
Jan Blease is deputy head of Northfleet School for Girls in Kent.