Is there a way for me to change the student grade level for all students at once?

The Grade Level field on the Student Summary tab can be increased or decreased by one for all students at once by any User ID with the Students permission Clear Answers in Student User-defined Fields and Update Student Grades. 

From the Tools menu select Update Grade Levels.  Select whether to Increase Grade Levels by One or Decrease Grade Levels by One. It is not possible to change the Grade Level from, for example, 3rd Grade to 6th Grade; it can only be done 1 year at a time.