This exemption is allowable only once for students who have been enrolled in US school for fewer than 12 months. The time starts on the last day of the testing window for the PSSA ELA or Literacy Keystone Exam from the previous year. If a student enrolled in US schools for the first time after the last day of PSSA ELA or Keystone Literature testing the previous year, then he/she is eligible to be exempted from the test.  This is covered in the PSSA/Keystone testing accommodations manual.

From the manual:

The Every Student Succeeds Act (ESSA) signed into law on December 10, 2015, allows flexibility for EL students whose enrollment in a school in one of the 50 States in the United States or the District of Columbia fewer than 12 cumulative months (not consecutive) an option of taking the ELA PSSA or Literature Keystone exam. A student’s enrollment in a school in Puerto Rico is not to be considered as enrollment in a U.S. school. It is incumbent upon the LEA to determine a student’s status. The eligible student will have been enrolled in a US school less than or equal to 12 cumulative months by the end of the current school year PSSA ELA/Keystone Literature testing window.