Describing Middle School in european grant application
Describe it like you did here: 8th grade, and then in parenthesis put (ages 12-13) to remove any ambiguity. That will only cost you a couple of words, and no one can misunderstand.

To expand on @ORMapper commented, in Sweden middle school would be "mellanstadiet" which is grades 3-5, and the students are aged 9-11. So specify both what the gade was "called" and the age, and avoid descritors like "middle school"that can vary a lot by country.