Is the decline in academic achievement after repentance and commitment to worship considered a tribulation?

It is not unlikely that what you are suffering from after your repentance, in terms of poor academic achievement, is a waswas (whispering) from Satan to divert you from the path of righteousness, for he is a clear enemy to humankind. Your steadfastness in repentance is not measured by what befalls you, whether ease or hardship, for all of that is due to divine wisdom, so that the servant may complete his servitude to Allah in all circumstances. Therefore, remain steadfast in your repentance, pay no heed to the whisperings of Satan, and exert effort in your studies.