All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First off all, we should say that you are on much good. Therefore, you should continue, according to your ability, to recite the mention of Allaah, pray at night and recite the Quran. We hope that Allaah will accept your deeds and forgive you.
What is most important is that you should be perseverant in performing your religious obligations, the most important of which is the prayer, and stay away from all prohibitions.
Saying that you cannot devote to Allaah more than this time is wrong since you can devote all your time to Allaah, and this is easy for whomever Allaah made it easy. This is done by having a good intention, for instance you may intend by your studies to benefit and contribute to your Muslim nation, and this is a good intention for which a person is rewarded. Moreover, you may engage yourself in one of the greatest acts of worship which does not cause any hindrance to your studies or take any time from your studies; this act of worship is to mention Allaah earnestly with the tongue and heart when you finish your studies and at times when you do not study. Allaah Says (what means): {Verily, the remembrance of Allaah is more important and greater.} [Quran 29:45]. Furthermore, it is confirmed that the Prophet said: ''Shall I not tell you about the best of your deeds, and those that give you the highest rank, and those that are the purest with your Lord, and are better for you than giving gold and silver (in charity), and better for you than meeting your enemy and striking their necks or having your necks being stricken (in Jihaad)?" They said: "Please do, o, Prophet of Allaah." He said: "It is the mention of Allaah, the Most-High." [Ahmad and At-Tirmithi].
Therefore, you may mention Allaah earnestly when going to school and when coming back, and you should correct your intention regarding studying. By doing so, all your time will be devoted to Allaah. It is in this way that the entire life of a believer is devoted to Allaah. Allaah Says (what means): {Say (O Muhammad): "Verily, my prayer, my sacrifice, my living, and my dying are for Allaah, the Lord of the Alameen (mankind, jinns and all that exists).} [Quran 6:162-163].
To sum up, a good intention turns many permissible acts and customs like studying and sleeping and the like, into acts of worship for which one is rewarded by Allaah.
Allaah Knows best.