This honestly doesn't make a lot of sense to me. But it would be great if you can describe more about the problem you are trying to solve. But I've a few points to make
1. Why does a Child have a static field called age ? If each child object has an "age" property, it should be an instance variable.
2. I'm not too sure what you're doing in line 4 of the School class ? you don't need a age property for a school , and that too read from a static field of a Child.
3. Capitalize the first letter of your classes.
The answer about static fields is that you should avoid them as much as possible. As you have been told, there is only one copy of a static field per class, so making age static implies that all children are the same age.
You will occasionally find circumstances where static fields are appropriate, but I am sure this isn't one of them.