There are two variants to this prayer:-
O Great St. Joseph of Cupertino who while on earth did obtain from God the grace to be asked at your examination only the questions you knew, obtain for me a like favour in the examinations for which I am now preparing. In return I promise to make you known and cause you to be invoked.
Through Christ our Lord.
St. Joseph of Cupertino, Pray for us.
O St. Joseph of Cupertino who by your prayer obtained from God to be asked at your examination, the only preposition you knew. Grant that I may like you succeed in the (here mention the name of Examination eg. History paper I ) examination.
In return I promise to make you known and cause you to be invoked.
O St. Joseph of Cupertino pray for me
O Holy Ghost enlighten me
Our Lady of Good Studies pray for me
Sacred Head of Jesus, Seat of divine wisdom, enlighten me.
Another prayer that was amazing for me during my exams and always is the prayer to the holy spirit:-
PRAYER TO THE HOLY SPIRIT
Holy Spirit, you who makes me see every-thing and who shows me the way to reach my ideals, you who gives me the Divine gift to forgive and forget all that is done to me, and you who are in all the instincts of my life with me, I in this short dialogue, want to thank you for everything and affirm once more that I never want to be separated from you, no matter how great the material desires may be. I want to be with you and my loved ones in their perpetual glory. To that end and submitting to God's holy will, I ask from you..... etc. (your request).
(This prayer should be said for 3 consecutive days. After the 3rd day the request will be granted. While making this request should:-
Promise to publish it on granting of the prayer.
You should always remember to give thanks to God for every blessing you get in your life.
Hope this helps.