The youngest average first marriage age in the world is 19.2 in Chad, Karakura's average first marriage age is below that. Karakura is less progressive than Chad
Allow me to now support my point with real-world arguments that are completely unrelated to SchoolRP and hold no meaning.
A young first marriage age is indicative in most areas of a region that is behind on gender progressiveness. By entering matrimony at that age, neither partner has the experience necessary for surviving in the real world, or unlikely has the ability to identify a bad companion. This results in a higher than average level of domestic violence, abuse, or premature death. It also restricts the ability for either partner to pursue higher education, as the partners need to pay for living together and all that stuff.
Additionally, the Japanese school year ends in late March. This means that somebody born on January 1st will have less than 3 months to be married before they graduate. Seems logical
+1 Ignoring my irrelevant points, 18 year olds should not be getting married en-masse.