After someone stops going to school, "going back to school" means enrolling and re-starting their education.
For example, if you dropped out of high school and you're considering going back to get a high school degree, you can say this:
I'm thinking about going back to school.
You can also use this phrase if you graduated from a program, but you're going to continue on to a higher level of education.
After working for a few years, I went back to school and got my Master's degree.