A question about Japanese Schooling?

Question:

So, if i was to turn 18 in April this year, When would be my last year of highschool?
The school year that just ended in March or the upcoming/current school year?

Just Curious.
Thanks! :)

Answer:

If your birthday is April 1st, you graduated from a high school in March.

If your birthday is April 2nd or later, yes, you are now in the 3rd year of high school from this April.