uses
  // System.JSON,
  System.JSON.Types,
  System.JSON.Writers,
  System.JSON.Builders;

procedure TForm1.Button3Click(Sender: TObject);
var
  m_StringWriter: TStringWriter;
  m_JsonWriter: TJsonTextWriter;
  m_JsonBuilder: TJSONObjectBuilder;
begin
  m_StringWriter := TStringWriter.Create();
  m_JsonWriter := TJsonTextWriter.Create(m_StringWriter);
  m_JsonBuilder := TJSONObjectBuilder.Create(m_JsonWriter);
  try
    m_JsonWriter.Formatting := TJsonFormatting.Indented;
    //JSON构造
    m_JsonBuilder
      .BeginObject
        .BeginArray('五班')
          .BeginObject
            .Add('姓名', '张三')
            .Add('成绩', 75.5)
          .EndObject
          .BeginObject
            .Add('姓名', '李四')
            .Add('成绩', 21.7).
          EndObject
        .EndArray
      .EndObject;
    //输出
    Memo1.Lines.Clear;
    Memo1.Lines.Add(m_StringWriter.ToString);
  finally
    m_JsonBuilder.Free;
    m_JsonWriter.Free;
    m_StringWriter.Free;
  end;
end;