You may have to enter each grade one at a time pressing enter after each key press.  The reason is that in your readGrade() method you are most likely making a new Scanner instance every time.  Each instance of Scanner has its own input buffer, so the first time you want to read all of the scores you might enter them on one line which means they are stored in the input buffer of the first Scanner.  After you call readGrade() again that input buffer is gone which means all of those scores are also gone.

This is intended.  You will need to enter each score one at a time pressing enter after every score.  During the lab this week, we will discuss a way to deal with Scanner in multiple methods.