Chemical changes are irreversible.
A chemical change that could happen at school would be when you have a chemistry experiment and you mix an acid and a base such as mixing hydrochloric acid with sodium hydroxide.
A chemical change that could happen at home would be baking a cake. You can't take the ingredients used in baking a cake back apart because they have changed their chemical composition.
A chemical change that happens in nature could be a forest fire. The burning of wood would be the chemical change in this case.