{'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?