Solution

"Go back" is the correct expression to use when someone returns to a previous place, situation, or activity. In this context, it's used to indicate a return to teaching after a two-year gap. "Go off" is generally used for things like alarms or explosives, and it doesn't fit the context of returning to teaching. "Go along" typically means to accompany or proceed, but it doesn't convey the idea of returning to a previous activity. "Go on" usually means to continue, and it doesn't fit the context of returning after a gap.