You pay for these lessons with the basic codestones (Mau, Lu, Vo, Tai-Kai, Eo, Main, Zei, Orn, Har, Bri). Each time you train, you have to pay with random codestone(s) that you do not know until you get to the screen that shows you what codestones you have to pay for. These codestones can be found in random events around the site or you can purchase them through the shop wizard, trading post, or auctions.
The level you are corresponds to how many codestones you have to pay. The more your level, the more codestones it costs and the longer the lesson lasts for.
Levels 1-20: 1 Codestone Levels 21-40: 2 Codestones Levels 41-80: 3 Codestones Levels 81-100: 4 Codestones Levels 101-120: 5 Codestones Levels 121-150: 6 Codestones Levels 151-200: 7 Codestones Levels 201-250: 8 Codestones
After level 250, you have to go to the Ninja Training school also located in Mystery Island.
Note: Your agility, strength and defense can only be trained if all your stats are equal to or less than twice your level. Endurance, however, can be trained if your stats are equal to or less than thrice your level. On the other hand, your level can be trained at any time.
Level: 20, Strength: 46, Defense: 40, Agility: 30, Endurance: 42
Here, only endurance and level can be trained. To train the other stats, you will have to train your level to half the highest stat, i.e. 46/2 = 23.