You say: "On the other hand. Present Perfect is for a past event which has a result now. But if somebody was learning something in a school then the result is still there - a person remembers it."
You are exactly right. The bit you are missing (as learners often do) is that the choice of whether to use the perfect or not is not about the objective fact of whether or not the result is still there: it is about the speaker's choice of whether to bring that present relevance to the fore in their speech.
Choosing to use the perfect conveys that there is some present relevance to the events described, though the exact interpretation of that present relevance can vary: it might be that the event is very recent, that it attained a state that still holds, that it had a result that is relevant to the present discourse, or that it is seen as one of a series of events extending to the present. (In the last two cases, the event might actually be long ago).
But in most cases, the speaker can choose not to use the perfect in referring to exactly the same events, because they don't need to give prominence to that present relevance.
With your "I've reached the top": as somebody said, usually you would say that if you are still at the top, and say "I reached the top" when you are no longer there. But those are not the only possibilities. Consider:
Have you ever reached the top? Yes, I've reached the top twice: once seven years ago, and then again last year.
Both the question and the answer use the perfect for long-ago events, because they are choosing to treat the events as part of a series that potentially reaches to the present. Another answer might be
Yes, I reached the top last year.
The speaker is now talking about a single event, and is not focussing on the continued range of times in which they might have reached the top, so they use the simple past. (For that reason it is unusual to use the perfect with a specified point of time in the past).
But now consider the following conversation:
"I've reached the top!" "That's impressive! How did you reach the top?"
The perfect in the first sentence implies that the speaker is still at the top (that's not the only possible interpretation, but it's a likely one). But the question uses the simple past, because they are asking about an event which is in the past, and have no need to focus on its present relevance. They could have answered "How have you reached the top?", but that feels less likely to me, exactly for that reason: it would be giving focus to something that is not relevant to the particular question.