Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Satan is keen on misguiding Muslims and leading them astray. And when a person commits a sin and repents from committing it, Satan would come to him again and tries his best to make him commit that sin again.
Allah says: "Ever feeble indeed is the plot of Shaitan (Satan). (An-Nisa 4:76)
Therefore, if a person endeavors to repel the plots and whispers of Satan, he/she will indeed succeed.
The following are some matters that help avoid falling into sin:
1. To feel that Allah is always looking and watching the person, and to think about His greatness.
2. Not to follow one's thoughts and desires and other plots of Satan.
Allah says: " O you who believe! Follow not the footsteps of Shaitan (Satan). And whosoever follows the footsteps of Shaitan (Satan), then, verily he commands Al-Fahshâ' [i.e. to commit indecency (illegal sexual intercourse, etc.),and Al-Munkar [disbelief and polytheism (i.e. to do evil and wicked deeds; to speak or to do what is forbidden in Islam, etc.)(An-Nur 24:21)
3. To remember death as it can come suddenly. So if a person thinks about this, he will fear committing sins.
4. To supplicate Allah with insistence to grant refuge from Satan and his whispers, and from committing sins.
5. To repent: when a person commits a sin, he/she should quickly repent to Allah.
The Prophet (Sallallahu Alaihi wa Sallam) said: "Indeed all of the children of Adam commit sins, but the best of them are those who repent." Reported by Al-Thirmidhi and Ibn Majah.
Thinking that you do not get good results in exams because of your sins, is a correct thought. The Prophet (Sallallahu Alaihi wa Sallam) said: "Indeed a man is prevented from a provision because of a sin that he commits." Reported by Ibn Majah in his Sahih from the Hadith narrated by Thawbaan, may Allah be pleased with him?
As regards, Fajr prayer, you have to take the means that will help you pray on time:
1. Go to bed early.
2. Set the alarm clock.
3. Tell someone to wake you up for the prayer by phoning you, for example.
4. Avoid exhaustion during the day by doing very hard work.
5. Sleep in the afternoon for a short while (siesta).
If you take these means, you will wake up for the prayer, and if you oversleep sometimes after taking these necessary means, then, Insha Allah, you are not sinful. In such case the Prophet (Sallallahu Alaihi wa Sallam) said: "There is no negligence in sleeping" reported by Muslim from the Hadith narrated by Abu Qatada.
However, if you have the habit of oversleeping and not praying Fajr, and you do not take necessary means to wake up, then this is forbidden.
The Prophet (Sallallahu Alaihi wa Sallam) said: "Fajr and Isha prayers are indeed the most difficult prayers for the hypocrites." Reported by Bukhari, from the Hadith narrated by Abu Hurairah.
Allah knows best.