good grades

Making only Du`a’ to Obtain Good Results in the Exams

asalamualaykum. i have just recently finished my exams and will be going to college in september inshallah. i have had a question that has been on my mind for a while now. i have a couple of friends approximately 6 who are all also muslim, the problem is though that they dont act like muslims. they do very sinful acts like drinking, smoking drugs and getting very friendly with girls. they are all aware that these are sinful acts but do not stop. their parents do not know. befor my exams had started i worked very hard in my studies and prayed and made duas to make the exams easy for me. my friends however, they didnt pray and make dua and just carried on with the sinful acts. however, when it comes to exams they do not have to revise and work as hard as i do, but then when the results come the achieve the top grades with out making dua and praying, also they do not study as much and still achieve the top grades. now recently i have just done my important exams and made lots of duas for the exams to make them easy for me, but i found them difficult and i have a feeling i will not do very well. i cannot understand why this could happen as my friends who do the sinful acts and never pray, they found the exams very easy and they are not worried about the results they will get. and i think they will achieve the top grades. can you please explain to me why this is. alhamdulillah i know my rights and wrongs in islam and i try to stay away from sinful acts but i cannot understand why some one who is embarassed to pray and be a muslim still achieves the top grades and gets into the top colleges without making duas. is there specificly any duas i can read until my exa ms restuls come? Jazakullah for your time and effort.