(someone) up to monkey tricks

Translations
(кто-то) занимается мошенничеством
We caught him up to monkey tricks, trying to change his grade.
Мы поймали его на мошенничестве, пытающегося изменить свою оценку.
Definitions
The phrase '(someone) up to monkey tricks' means that a person is behaving mischievously, playfully causing trouble, or engaging in sneaky or naughty activities, often in a lighthearted or humorous way.
Words
someone

pronoun
кто-то
up

adverb
занимающийся
to

preposition
в
monkey

adjective
обезьяньи
tricks

noun
проделки