Make a common event and set it to parallel process. Turn on the switch in the game that you set as precondition for the parallel process. Now store a character stat in a variable and check if the variable is equal or higher than the value your hero needs to learn the stat. If yes, let him learn the stat. Put as many variable storing/checking and skill learning for as many heroes you want in that parallel process. You can find the common event window inside the Database.