What is the ruling on buying sweets for friends after agreeing to that as a condition for whoever gets the lowest grades in the exam?

If the exam is in a field that benefits the Ummah, there is no harm in receiving a financial prize for it. Some scholars have permitted competitions with a prize in academic matters. The described scenario is not gambling if the purchase of sweets is a voluntary act and not an obligation. Sheikh Ibn Jibreen issued a fatwa (definition) permitting the losing team to provide dinner for the winning team if it is done voluntarily and out of generosity, not as an obligation, clarifying that it does not fall under gambling as long as it does not result in significant financial loss.