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