Dear <school>,
My son found £20. I wanted to teach him good ethics and honesty by getting him to hand it into the lost property.
No one has came forward to claim the £20, so my son should be rewarded for doing the right thing by taking ownership of the £20.
You, however, have decided to keep the £20. Which by all definitions is theft.
This teaches my son that in future he should just not tell anyone and run home with the money, and that theft is the way to get what you want, which is the unethical and dishonest thing to do.
You are a school, think about this.
Regards,
—-
And CC in the headteacher if you have their email, and the local council governing body of education.