What is the ruling on one who vowed to pray two rak'ahs every time he achieved a good grade in school exams, then failed to do so and wished to abandon this vow?

A conditional vow is disliked and does not advance or delay anything. Nevertheless, it must be fulfilled if the condition upon which it was made is met. So, if you achieved a good grade, you are obligated to pray two rak'ahs. If you fail to fulfill the vow, the missed prayer remains a debt upon you and you must make it up. Some scholars have mandated an expiation along with making it up.