回答
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Your hair has to be less than 5 mm long and you cannot dye it.
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Your hair has to be cut less than 5 mm long and you are not allowed to dye it.
学校の校則を説明する時に「髪は5mm以下にカットし、染めてはいけない」は英語に訳したいならこのようになります。
Your hair has to be less than 5 mm long and you cannot dye it.
Your hair has to be cut less than 5 mm long and you are not allowed to dye it.
ご参考までに!