Size 6 or size 7 which ball does your area use ?
What size ball shall be used for students (boys and girls) in grade 7 and eight. (12-14 yrs) If different from high school competition, what advantage would the students gain from a different size ball ?

Inches
 

In Michigan, 7th-8th and probably lower use the 28.5" circumference.
We call it a women's and Youth ball.

Size 6 or 7? Never heard of them.

Women/Girls HS 28.5"
Men/Boy HS 29.5"

All else varies. Some youth leagues use the women's size for both genders.

There's a junior size (27.7) but I've never seen it used in a game.

There's even a child's size.

Perhaps he's looking for the soccer board! ;)

Full size in soccer is a 5. Playing with a 7 would be like trying to score with a medicine ball!