Why did I go back to school after thirty years? How do I explain all that without going into many paragraphs of explanation?
It may be helpful to write everything out, in many paragraphs of explanation, first. Once you've written your answer out, you can look at it, or have a friend or colleague help you, and decide what is the most important and pertinent information in your long answer. Then you will be able to distill your long answer into a more concise, informative one.
You may want to first consider why you stopped going to school, whether it was high school or college. Explain the circumstances that lead to the end of your schooling. Second, look at the key factors in your life that caused you to regret or rethink your decision about ending your schooling. Finally, think about what was the event that made you finally decide to make returning to school a priority. While I'm sure the answer is a complicated and long one, it is important to consider what the key points are, and highlight those, rather than construct an autobiography for your essay.
The above answer by sdesabato is excellent. I agree with it entirely--especially about writing everything out in many paragraphs and then selecting the best for your final draft.
I have one other suggestion which might make your job easier. You might begin with writing about how you felt when you were back in school after all those years. If it felt good to be back, that should have proved you made the right decision. Then you might explain how and why you made the decision.
I suspect that you found school was a lot more interesting and enjoyable after being away from it for so long.