Heiburg-ke Sannan wa Koakuma Shota desu the Comic

Selphil is the youngest child of the Duke of Heiburg, one of the most powerful families in the country.

In reality, he is a reincarnated person from modern Japan! Selphil lives skillfully using his cute, angelic appearance and knowledge of modern Japan, but an incident occurs at the school graduation party he attends on behalf of the Duke...?

https://www.cmoa.jp/title/308883/