A High Score can be obtained by beating your own personal high score in the hidden scenes.
You can only receive 1 high score per scene. Ex. If you get 650,000 in Ch 25-2 and then get 652,000 in Ch 25-2 again, you only receive ONE high score on the leader board as it is still the same scene.
You will only move up in rank if your score passes the person above you. If the person below you passes your current score, you will fall to the next rank below.