Mathematicians cannot be properly classified into middle school students, high school students, undergraduates, postdocs, tenure track job holders, etc.  The learning advice depends on the person's knowledge, experience, and level of understanding of the subject, not on his formal position and career stage.  (It also depends on what he finds interesting and aesthetically appealing, of course.)  The mathematical level of high school students varies so much that no general advice based on the "high schooler" status is possible.