It was about a crown prince and noble (?) girl’s love story. The girl might’ve been a reincarnater. They mutually agree on the arranged engagement, to end up loving each other later.
The girl is a bit quirky, because every time she comes over to talk to the prince, their guards have to restrain from laughing. They called it an ab workout or something like that.
There was also an arc of a knight x princess novel going around, and the crown princess and her knight did not like the representation of a knight’s vow.
The crown prince and princess also go to school, where their age and height difference gives rumors to the prince being a lolicon.
Please help me find this, it might be a bit of an older manga.