these duas are for anytime...so you can use them for exams as well...
1. "Yaa Hayyu Yaa Qayyoom bi rahmatika astaghees
(O Ever Living, O Sustainer, by Your mercy I seek help).
2. Allahumma la sahla illama ja-'altahu sahla wa anta taj 'alul hazana eza shi'ta sahla
O Allah! Nothing is easy except what You have made easy. If You wish, You can make the difficult easy.
PS: lets be better muslims for duas to work like pray namaaz, respect parents, no dating/affairs, no alcohol, no hurting others, etc... (not only during exams but whole of our lives.)
Best of Luck :)