How can a student deter his male and female colleagues from deviance, immodesty, and immorality at the institute, given the presence of only one assistant?

To repel your colleagues from deviation, seek Allah's help and supplicate for their guidance, as the Messenger of Allah, peace and blessings be upon him, did. Invite them with gentleness and wisdom, starting with the most intelligent among them, and explain to them the consequences of deviation. Be humble and compassionate, and know that the path of Dawah (inviting to Islam) is fraught with harm, so do not despair. Remember that those who adhere to their religion in times of tribulation are the strangers who rectify when people become corrupt, and they are a group manifest upon the truth.