What a blessing. May you rejoice in it. The short answer is: all of them! Even a book like Leviticus that people have poorly informed aversions to has important lessons for young (and old) people - Nancy Gatz' commentary for children is helpful on this if you're looking for a challenge, or the wonderful Dutch commentary series called "Promise and Deliverance" S.G. DeGraf.
However, depending on their maturity and understanding of the Gospel, here are some that might be helpful to consider in consultation with your elders:
1. Gospel of Mark - short and accessible, I always recommend Mark to people that I'm evangelizing or beginning to disciple. I think it's the best one to start with. It's short enough that you could cover it in a school year as well.
2. If the majority of them are professing Christians and are looking to delve more into the word, then, Romans is a good choice. Not only is it a great summary of the Gospel it is an outline of all of the great doctrines of Scripture. It can also feed at multiple levels (basic Gospel like Romans 6:23 but also the blessings of the Covenant and the importance of the doctrine of Sovereignty - chapter 9)
3. Some great and helpful OT books if they're unfamiliar with the glory of the OT - Genesis (great for practically equipping them for high school biology/history class if they are in the public system, also basic apologetics (pre-interpreted universe - see Genesis 1, and laying out the Gospel - Creation, Fall, Promise of Redemption (Genesis, 1-3 and particularly Gen. 3:15), Covenant 15:6 etc. - I love Genesis and if I hadn't preached through it recently I'd be doing it with our youth). It's a great opportunity to introduce them to the true hero of the Gospel (God) through the failures of the patriarchs. Another good book to consider in the OT would be Ecclesiastes to point to the vanity and emptiness of the world, the joy of youth with the light of the Gospel - teach them to love a verse like 11:9 "Rejoice, O young man, in your youth, and let your heart cheer you in the days of your youth. Walk in the ways of your heart and the sight of your eyes. But know that for all these things God will bring you into judgment." ).
Delight in your opportunity to share these treasures of Christ with these young men and women, J.R. and may the Lord encourage you as you seek Him and point others to His amazing glory