Absence is the state of not being someplace, like school. It can also refer to the amount of time something has been, or plans to be gone.
We most commonly think of absence as the opposite of present in places school, but really it can refer to anything that's missing or something that's lacking. Darkness is an absence of light, and cold is an absence of heat. The figure of speech "absence makes the heart grow fonder" means "when my beloved isn't here, my feelings for them get even stronger" or "when something's gone, you want it even more."