Wuppas, this is just my opinion, however, I think your class my be asking the right questions at the wrong time. This may help you understand what I mean.
Before, looking for scriptural refrences to “prove” the Sacraments, I think you should discuss what exactly is a Scarament (s). They should understand that inorder to understand what a sacrament is they should understand the Nature of the Church. I am sure, based of Paul’s teachings on the Church, there should be no denying that the Church is the Mystical Body of Christ - through the Church, Christ is present and active here and now. The Church is the “visible” sign of Christ’s presence.
The Catechism, puts it much better than I can, but Vat II taught that Christ is present and working through the Church. A Sacrament, then, is a Symbolic (Visible), Saving act of Christ, in and through the Church. Christ saving actions are made visible through the actions of the Church.This includes those 7 specific actions of Christ through the Church - the 7 Sacraments.
This “Sacramental” understanding of the Church,in general and our Living Faith Response in particular, is crucial in their understanding of what they would be initiated into through the RCIA. God/Christ becomes incarnate in and through the Church’s and our actions (and 'God forbid, this means ‘Good Works’).
If your class comes to this understanding about the sacramental nature of the Church, and this is constantly re-enforced, I think it may be easier to present the “Biblical Proof” when studying each of the 7 Sacraments.
Finally, I think you may also remind your class that not everthing Christians believe are found spicifically in Scripture in Chapter and Verse. More often than not, God’s revelation (That is how God reveals himself and His Nature) comes to us gradually and is realized not only through the Scriptures but through the Church’s liturgical life, Studies, Councils, teachings - all part of the Church’s Apolostolic Tradition. Should your class have some difficulties with this (which is only natural given their back groung and life experiece) talk about the Sacrament of Baptism and its relationship to our understanding of the Trinity (a word not found in the Bible but believed by most Christians and all Catholics).