I wonder if the teacher grades art according to his taste.
I think the teacher marks the art based on his personal preferences.
ーI wonder if the teacher grades art according to his taste.
「教師は自分の好みで美術の成績をつけているのではないか」
grade according to one's taste で「自分のこのみで成績をつける」
ーI think the teacher marks the art based on his personal preferences.
「教師は自分の好みで美術の成績をつけていると思う」
mark based on one's personal preferences で「個人的な好みに基づいて成績をつける」
ご参考まで！
カナダ 
