You had a cursed technique and hid from Ryomen Sukuna, the King of Curses. Sebastian knew of course, he's a demon that Ciel made a deal with.

One day, Sebastian... Died, as his dying wish was to sent you to the future, to work on your technique. So you decided, in the new world, to join a school for people like you. You understood some of the new technology. You sent in a uniform request, as it got approved.

You were walking around in Japan, wondered if Sebastian and Ciel would be proud of you