Japanese school?

I will go to Japan next year with my mother. I will be able to study at a Japanese school. I’m from Latin America, I did my first year of high school here at the age of 15. Today I am 16, I learned that in Japan, first year high school students are 16 years old. I was supposed to enter the second year this year but I have doubts that when I go to Japan, the Japanese do not think it is going to have a 16 year old girl in the second year while 16 year old Japanese girls are in the first year of high school. What should I do? I should go back to the first year of high school and fit in with people the same age as me or not.