"upper sixth" meaning in English

{
  "head_templates": [
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      "args": {
        "1": "-"
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      "expansion": "upper sixth (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "British English",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
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          "source": "w"
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          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
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        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Scottish English",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Year 13, the second year of sixth form for 17 to 18 year olds, in which students sit A-levels."
      ],
      "id": "en-upper_sixth-en-noun-dDnfcFSd",
      "links": [
        [
          "sixth form",
          "sixth form"
        ],
        [
          "A-level",
          "A-level"
        ]
      ],
      "qualifier": "UK except Scotland",
      "raw_glosses": [
        "(UK except Scotland) Year 13, the second year of sixth form for 17 to 18 year olds, in which students sit A-levels."
      ],
      "related": [
        {
          "word": "lower sixth"
        },
        {
          "word": "sixth form"
        }
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "word": "upper sixth"
}

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "upper sixth (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "lower sixth"
    },
    {
      "word": "sixth form"
    }
  ],
  "senses": [
    {
      "categories": [
        "British English",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "Scottish English"
      ],
      "glosses": [
        "Year 13, the second year of sixth form for 17 to 18 year olds, in which students sit A-levels."
      ],
      "links": [
        [
          "sixth form",
          "sixth form"
        ],
        [
          "A-level",
          "A-level"
        ]
      ],
      "qualifier": "UK except Scotland",
      "raw_glosses": [
        "(UK except Scotland) Year 13, the second year of sixth form for 17 to 18 year olds, in which students sit A-levels."
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "word": "upper sixth"
}