print(df)
[{'GROUP': 'AAA', 'AVG_GRADE': [[7.0, 5.5], [8.0, 6.5], [9.0, 4.5]],
    'AVG_MINUTES': [[7, 67], [8, 58], [9, 55]]}, {'GROUP': 'BBB',
    'AVG_GRADE': [[7.0, 5.1], [8.0, 5.4]], 'AVG_MINUTES': [[7, 15], [8, 18]
    ]}, {'GROUP': 'CCC', 'AVG_GRADE': [[9.0, 5.5]], 'AVG_MINUTES': [[9, 34]]}]
How to convert dataframe to a json string and apply some transformations to this string?