Graduate
A person puts a lid on their cross after they graduate. It’s a symbolic gesture, putting the lid over the cross, but it’s just what you do when you graduate around these parts.
そつ
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Oops, looks like you forgot your lid. Without the lid, you’ll be held back. So two (そつ) more years now until you can graduate. So two more years, eh? Just for forgetting a lid? Nonsense!