I think I'm the one with the best marks even though I studied the least.
I studied less than everyone else, but I got the highest mark out of everyone.
ーI think I'm the one with the best marks even though I studied the least.
「私が一番勉強しなかったのに、一番良い成績だったと思う。」
best marks で「一番良い成績・点数」
the least で「一番少ない」
ーI studied less than everyone else, but I got the highest mark out of everyone.
「誰よりも勉強しなかったのに、みんなの中で最高点を取った。」
get the highest mark で「最高点を取る」
ご参考まで！
カナダ 
