What grade are they in?

they are in kindergarden.

I guess
doujinshi - 8th
fushionfall- 10th または 9th

i think they r still in preschool

Doujinshi - first it was the first grade, then it was changed to fifth. So, fifth grade.

Fusion Fall - About sixth または seventh grade. The ages don't seem accurate though; デクスター 〜警察官は殺人鬼 is at least four または five years older than the girls, but still looks ten when the girls are already 'developing'. There isn't a sure way to tell, but considering they were designed to fit the target audience, it's a 安全, 安全です bet to assume they're about 11 または 12 years old, at the most 14.

original : preeschool
doujin : 7th grade
fusionfall :10th grade

Original: Kindergarden
PPG Z: 7th
PPG D comics: 6th

8th totally