Enroll
People had to ride their bamboo plows long ago to enroll in things. There were no cars yet, so they would hitch a cow to their bamboo plow, and dig their way to the government office or school to enroll.
Picture the little hut that passed for school long ago. All of the bamboo plows are parked around it, as the people enroll themselves and their kids up for classes.
せき
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If you enroll in something new people will find you very sexy (せき). This was true long ago too. There's nothing sexier than trying to improve yourself.
Picture a line of people enrolling for new classes, sports, citizenship, etc. The moment they've enrolled they poof into the perfect, sexy person (in your eyes).