What you said! ^ ^ ^ And get them to Trevi Fountain and snap their pictures throwing coins. You may have to take a few taxis along the way and put up with some overcharging, but it shouldn't be a really significant problem with a metered taxi. I wouldn't recommend the metro for teens seeing Rome the first time. Taking taxis would allow them to see the city rather than primarily the metro.

Toss the coins at Trevi Fountain and snap the pictures. Assume that they'll find their way back to Rome someday and wait in line for the "must sees" ("must sees" for adults, that is). In the meantime, let them see something of the spirit of Rome. (I don't mean Rome as in crossing busy streets and dodging Vespis, I mean Rome as in going to look at fountains--you know.) You can tell your taxi to drive past the Colosseum on your way to whatever and point out the window: "Look, guys! The Colosseum!" If they want to stop and stand in line for a tour (by tour, I mean self-propelled, as in, your own two feet), they will let you know.

Best of all: Have their pictures snapped with some of the costumed "soldiers" that are standing around outside the Colosseum for picture-taking opportunities. Price--gratuity, whatever-- ought to be the equivalent of a few dollars. The teens will have something to show their friends back home. (If you see that the ticket line is short, you can always buy tickets and go inside and walk around for awhile, but if the ticket line is long, they've got their memories anyway.)

Then there is the Forum, which I would rank second, after the Vatican/Sistine Chapel and St. Peter's basilica--ahead of the Colosseum anyway. (Vatican City first, Forum second, Colosseum third.) But the Forum takes energy and time and an interest in the subject, so much energy that with a group, you'd be facing a Mutiny-on-the-Bounty situation.

I hope that advice doesn't sound superficial, but seeing either the Vatican or the Forum is a major effort that will take the entire day and leave you and them exhausted. The Colosseum can be done easily if there are no lines. But there is so much overview of Rome that can be done instead, on a first visit, for those who will presumably be back someday. . . .

The Vatican is wonderful, unforgettable, a reason itself to see Rome but a lot of exhausting work, and you just don't have the time, and a group would not have the energy or focus that getting to the Sistine Chapel would entail.