Is intentionally lowering one's grades to appear unsuccessful before peers—in order to avoid their envy and requests for cheating—considered a sin or a form of lying and deception?

What you mentioned about intentionally making mistakes and showing yourself to be a failure in studies is not deception, but rather harming yourself and your parents. Harming oneself or others is forbidden, as the Prophet (peace be upon him) said: "There should be no harm nor reciprocating harm." This approach is not a solution to the envy you face from your colleagues. Do not let that deter you from striving and excelling in your studies, while not helping others in forbidden cheating. You should endeavor to overcome your fear of your colleagues, seeking Allah's help.