Many Hindu youngsters have this doubt as to which vrat can guarantee success in exam. The best vrat for success in exam is the monthly Sankashti Chaturthi Vrat dedicated to Baghvan Ganesha (Ganpati).
All kinds of impediments and troubles regarding studies will be alleviated by Ganesha who is Vighnaharta.
Sankashti chaturthi falls on the fourth day after the full moon day. Moon rise is to be present during the Sankashti chaturthi tithi. So the tithi can happen a day earlier too.
The vrat is from sunrise to sighting of the moon or moonrise time in your area. Apart from fasting on the day, devotees visit Ganesha temple and offer prayers. Feeding animals on the day is meritorious.