I teach in a Catholic Classical High School so we certainly what to cover the significance of the redemptive theme of the Inferno (as though that were possible to miss!)
Indeed, you are correct in asserting that the "Paradiso" component of Dante has to be addressed. I might pose a different approach to teaching it, if you cannot teach the whole thing. Perhaps, addressing the historical context of Dante might be appropriate. This would allow you the chance to examine the work as both a work of spiritual and political value. Dante is mindful of the political context in which he is writing, which is why many people in the different levels of the inferno are actual people in Dante's setting. This might give students a background into the understanding of the work and then analyzing the different levels of the inferno can help reemphasize this. I think that if you can get through the ideas of how Dante classifies and categorizes hell, this might be very resonant with first year high school students. In this process, you can also bring in specific Cantos that you think are significant while students will still be able the full effect of the work. Another interesting application would be for students to create a modern version of Dante's work. If placed in a modern setting, who would be Virgil? Who would occupy the different stratas? Would new levels of hell be needed to be invented? I have always thought that this is a fairly interesting assignment to give to students in the composition of their own take off on the Inferno.
Since lust and suicide are two sins that are likely to be relevant to teenagers, I would suggest that you include Canto V and XIII.