I will definitely pray for you and that person at school
And as advice for ministering to him, you could give him maybe give him some christian tracks with verses on it (notably verses on drugs), like the following:
I Corinthians 15:33
Do not be deceived: “Bad company ruins good morals.”
I Corinthians 10:13
No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it.
Romans 14:1-3
As for the one who is weak in faith, welcome him, but not to quarrel over opinions. One person believes he may eat anything, while the weak person eats only vegetables. Let not the one who eats despise the one who abstains, and let not the one who abstains pass judgment on the one who eats, for God has welcomed him. Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand.
This last verse is a good one to keep in your own thoughts, as you critique someone, to not point the finger exactly, but lead him out of trouble that he is in.
2 Peter 3:9
The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance.
Remember this verse, and understand that if he doesn't take root to what you are witnessing to him, then keep in mind that you planted a seed, and that overall God is in control, and doesn't want to see anyone perish.