Katekyo Hitman Reborn!, The Breaker Recommendations
-In The Breaker it's clans but in Hitman Reborn! it's a mafia family.
-Main characters have to train their powers
-Main Characters are dragged into a world unknown to others. (Mafia and Murim)
-Both have an excellent teacher
-Both characters are wimpy at first, but grow to be more mature.
both shows have
a week protogonist that becomes a badass late in series
both protogonist have an overpowered teacher with a hidden past
both protogonist are dragged into a world unknown to them( mafia in hitman reborn and mirium in the breaker
both protogonist are school students
both have clans/mafia family
both protogonist have a strong bond with their teacher
both are a must read