Walaikum Asalam Warahmatullahi Wabarakatuh, May Allah keep you firm and there is nothing to be afraid about. At my school the brothers usually talk about the topic that speaks about the youth such as; surah Khaf, what it means to enjoy good and forbid evil, the life of Musab Ibn Umayr the young ambassador, Fear Allah wherever you are, the effect of using foal language, lowering your gaze, Among the people that will be under the shade of Allah on the day of reckoning is a youth that grow up practicing the deen. Why boyfriend/girlfriend is haram in Islam?, How to avoid sheytan's temptation, flee to Allah, The merit of following the sunnah and act... The topic various especially if you have non-Muslim teachers standing to monitor the place, they sometimes listen to the khutba and many of them convert because of the Akhlaq of the youth and your words. If you have Muslim student Union, discuse with them about the topic. So, in'sha'Allah with ikhlas and actually preaching about what you do you will do fine. Tawkal and start with praisng Allah, Sending salutation on the prophet in English or what ever the language. As for the Du'a, Do like Anas radiAllahu Anhu the kadim of the prohhet do which is Rabana Atina fiduniya hasaatan wafil akhirati hasanatan waqina azaban nar.. Surah Baqara, Which means Oh our lord give us good in this world, give us good in the hereafter and give us salvation from hellfire. Anas may Allah be pleased with him would make du'a whenever people ask him to do to the point where they said to him, Oh Anas do you not know any other du'a beside this du'a and he said what more do you want if you have good in the world, in the hereafter and you are freed form hellfire. Don't forget to invite your non-Muslim friend over.
May Allah make it easy for you.
