I am not a teacher.
The question should be, "How do you/does one pronounce 's'?" or, "How is 's' pronounced?"
The 's' of 'is' is always pronounced 'z' regardless of the letter that begins the following word.
In "last year" the 's' doesn't come before the 'y', it comes before a 't' and has the normal sibilant 's' sound you find in "seaside".
Student or Learner