A good example showing teamwork and its effects in a school setting is that of a school team. For instance, there could be a club at school dedicated to philanthropy or tutoring. This hypothetical group could work together to do good deeds (like clean up areas, work at food banks, etc. etc.).
If this is a more realistic situation, or more specific to your school, you could use the example of a group project. Teamwork is essential there in order to get the work done.