Is what I did considered oppression and concealment of the truth, and what should I do regarding my role in increasing my colleague's punishment, knowing that I am remorseful and most likely was an adult at the time?

We hope you will not be held accountable for what you did, as you may not have reached puberty, and because you did not intend to harm the child. However, you must repel harm from the oppressed if you are able and if it does not result in harm to you, for the Prophet, peace and blessings be upon him, said: "Help your brother, be he an oppressor or an oppressed," and he said: "No man abandons a Muslim man in a situation where his sanctity is violated and his honor is demeaned, but Allah, the Mighty and Majestic, will abandon him in a situation where he loves to be helped." And helping a Muslim is an obligation, whether by word or deed.