When I was in high school I somehow bullied those students who were juniors and now I really regret for what I did. I could not find them to ask them for forgiveness. Can I ask Allaah directly to forgive me?

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.

If the injustice that you caused to your colleagues is about material rights, like having taken money or belongings from them, or caused them a harm that necessitates a corporal punishment or requires blood money [Diyyah], and the like, then these material obligations should be given back to their owners or one should seek forgiveness from them, as the  Prophet said: "Whoever has wronged his brother, should ask for his pardon (before his death), as (in the Hereafter) there will be neither a Deenaar nor a Dirham (i.e. payment will not be in the form of wealth). (He should secure pardon in this life) before some of his good deeds are taken [according to the injustice that he caused to his brother] and paid to his brother, or, if he has done no good deeds, some of the bad deeds of his brother are taken to be loaded on him (in the Hereafter)." [Al-Bukhari]

So if you are unable to give them back their rights because of not knowing where they are, then give them in charity and intend the reward for them.

Nonetheless, if your injustice to them did not involve any money and the like, but it is only incorporeal rights like hitting them or insulting them and the like, then you only have to repent if you are unable to find them. The fact that you regret what you did is evidence that you repented to Allaah, as the Prophet said: "Regretting is repenting." [Ahmed] Besides, you should seek Allaah's Forgiveness as Allaah Says (which means): {And seek Forgiveness of Allaah. Verily, Allaah is Oft-Forgiving, Most-Merciful}[Quran 2:199]. Also, you can supplicate for your colleagues whom you wronged and ask Allaah to forgive them, as by doing so, it would be as if you have given them back some of their rights.  

Allaah Knows best.