An art installation is a piece of art which is three-dimensional, speaks to the feeling of the space, and transforms the space in general.
Therefore, since you are creating an art installation for your school, and the theme or idea is "good and bad," the piece needs to speak to the good and bad things one finds in schools.
There are many good things in schools. You could create something which speaks to any of the following:
-building positive character
-the importance of education
-content specific subjects (such as math, science, and/or literature, history)
-extra-curricular activities (sports, music, arts, and/or theater)
Unfortunately, some "bad" things exist in school. You could use some/any of the following to represent the bad things which happen in school.
I would suggest limiting the number of "bad" things (or paralleling them with good things so as to show hope).
Below are examples of art installations.
I loved working with the idea of juxtaposition when I was in high school. So it's something i'm very familiar working with.
When you think of good, you think of justice, honesty, and innocence. However, when you think of bad, you think of injustice, dishonesty, and guilty.
You wanna take two completely different ideas, beliefs, or actions and put them side to side.
My favorite was doing heaven versus hell or justice versus crime. Something with a heavy, deep meaning that will leave your audience with a deep impression and get them thinking!