اللهُمَّ لا سَهْلَ إلا مَا جَعَلتَهُ سَهْلا وَ أنتَ تَجْعَلُ الحزْنَ إذا شِئْتَ سَهْلا
Allahumma la sahla illa ma ja’altahu sahla, wa ‘anta taj-alul hazna idha shi’ta sahla
Meaning: "O Allah! There is nothing easy except what You make easy, and You make the difficult easy if it be Your Will."

Allahumma inii as'aluka fahmal-nabiyyen wa hifthal mursaleen al-muqarrabeen.
O Allah! I ask You for the understanding of the prophets and the memory of the messengers, and those nearest to You.

Allahumma ijal leesanee 'amiran bi thikrika wa qalbi bi khashyatika.
O Allah! Make my tongue full of Your remembrance, and my heart with consciousness of You.

Innaka 'ala ma-tasha'-u qadeer wa anta hasbun-allahu wa na'mal wakeel.
(O Allah!) You do whatever You wish, and You are my Availer and Protector and the best of aid.

Dua After Studying:
Allahhumma inni astaodeeuka ma qara'tu wama hafaz-tu. Faradduhu 'allaya inda hagati elayhi.Innaka 'ala ma-tasha'-u qadeer wa anta hasbeeya wa na'mal wakeel.

O Allah! I entrust You with what I have read and I have studied. (O Allah!) Bring it back to me when I am in need of it.
(O Allah!) You do whatever You wish, and You are my Availer and Protector and the best of aid.

Dua While Studying Something Difficult:
Allahumma la sahla illama ja-'altahu sahla wa anta taj 'alu al hazana etha shi'ta sahla.

O Allah! Nothing is easy except what You have made easy. If You wish, You can make the difficult easy.

There are ahaadeeth which indicate that this is to be said at times of hardship and difficulty. All of that includes exams and other difficulties.

1 – It was narrated that Anas ibn Maalik (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever says – when leaving his house – Bismillaah, tawakkaltu ‘ala Allaah, laa hawla wa laa quwwata illa Billaah (In the name of Allaah, I put my trust in Allaah, there is no power and no strength except with Allaah), it will be said to him: You are taken care of and you are protected, and the Shaytaan will keep away from him.”

2 – It was also narrated that Anas ibn Maalik (may Allaah be pleased with him) said: If something upset him, the Prophet (peace and blessings of Allaah be upon him) would say: “Yaa Hayyu yaa Qayyoom bi rahmatika astagheeth (O Ever Living, O Sustainer, by Your mercy I seek help).”

3 – It was also narrated from Anas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “O Allaah, nothing is easy but that which You make easy and You can make hardship easy if You will.”

4 – It was narrated that Sa’d ibn Abi Waqqaas (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) said: “The prayer of Dhu’l-Noon when he was in the belly of the fish: ‘Laa ilaaha illa Anta, subhaanaka inni kuntu min al-zaalimeen (There is no god but You, glory to You, verily I was one of the wrongdoers).’ A Muslim never calls upon his Lord with these words concerning any matter, but his prayer is answered.”