The first comment after they tell their friends they have become a Christian is often: "Is Christianity just superstition, so how would you answer that with more than just "No"?
My experience with God and His power to change my life. After that it's simply a matter of living out the principles of God's kingdom in our lives. It will then become self-evident that Christianity is not a superstition.
I think any interaction in what you describe requires a lot of prayer and faith in God, as well as a close personal connection with God. The Holy Spirit is the only person who can give us the right words to say at the right time as He knows the hearts, lives, thinking, experiences, etc... of the person to whom we are witnessing. There's no other way to know exactly what to say than to just trust God for the words to say as they will be different with every person.
Jesus said the following:
Matthew 10:
[FONT="][/FONT][FONT="][SIZE=+1]17[/SIZE][/FONT][FONT="] And, take ye heed of men, for they will give you up to sanhedrims, and in their synagogues they will scourge you, [/FONT][FONT="][SIZE=+1]18[/SIZE][/FONT][FONT="] and before governors and kings ye shall be brought for my sake, for a testimony to them and to the nations. [/FONT][FONT="][SIZE=+1]19[/SIZE][/FONT][FONT="] `And whenever they may deliver you up, be not anxious how or what ye may speak, for it shall be given you in that hour what ye shall speak; [/FONT][FONT="][SIZE=+1]20[/SIZE][/FONT][FONT="] for ye are not the speakers, but the Spirit of your Father that is speaking in you. [/FONT]
Yes, I know the promise is given for a specific time and place, but I believe it is also applicable for instances like you are describing. It is the Holy Spirit that can give us the right words to say. Look at Peter's experience on the day of Pentacost. It was simple speech, but it was inspired by the Holy Spirit and set home in Peter's listeners as it cut them to the quick and they wanted to know how to get right with God.
I have had multiple experiences like this in my life where I'm talking to someone I don't really know, and something spiritual comes out of my mouth that has me thinking, where did that come from? And the other person will say, someone who apparently has no interest in God, I've been thinking about that and wondering about that. It has often turned into an off the cuff Bible study and the person walked away thanking me for my help. I wasn't looking to witness. It just happened I was in the right place at the right time and God used me. We can't use God, God has to use us. When it happens it's one of those things that makes my day like no other thing possibly can. I go away with my heart singing for days.
If your students want to witness, and they build a good relationship God they will find opportunities as God will prepare the hearts of those He is drawing to Him and if your students have a strong desire to help their fellow man to know God, that will happen for them.
Don't worry about soundbites. Focus on the relationship with God and trusting Him to do that which we cannot know how to do. God's word will not return unto Him void. It will accomplish what He sends it out to do.