Very often, if you can use a continuous construction, you can also use a simple construction: the difference is not in the objective circumstances or events, but entirely in how the speaker is choosing to think about the events: as a completed or a continuing activity.
In the question What xxx since you left school?, both options are completely natural.
What have you done since you left school? might be paraphrased as "Give me a list of the things you have done since you left school".
What have you been doing since you left school? might be more like "How have you occupied your time since you left school?"
The answer might be exactly the same in the two cases, or it might be different, reflecting that difference in focus.
In this case, following on from When did you leave school?, the simple perfect might be seen as a challenge, meaning something like "Tell me what you have achieved since you left school." But not necessarily - it depends on the tone of voice.