Atashi no Bambi

Haruko Nanase wasn't always the brightest student, but through sheer hard work she successfully entered a prestigious high school and is at the top of her class. While riding the train one day, she witnesses a fight involving a guy wearing the same uniform as her. When the fight is over, he demands that she pay him a fee for watching it but she escapes. The escape is not to last though, because the next day the guy turns out to be in the same class, and worse, his seat is next to hers.

(Source: Keijo Shoujo Scanlations)