I think the late Confirmation has everything to do with keeping attendance up at high school youth groups and full employment for youth ministers, and not much to do with whether the sacrament needs to be done at high school level to be understood or appreciated.
When I was on my parish’s confirmation team, although the program was two years long, I think the whole crux of it was accomplished in one weekend retreat with the Salesians. They taught the Gifts and Fruits of the Holy Spirit and whatever the kids needed to know to “get it” about what they were doing. One weekend, just like the “Engaged Encounter” which does marriage prep. If preparing for a lifetime of marriage can take only a weekend and a couple meetings with a priest, how come confirmation teams can hold a kid hostage for two years?
Well, it appears that's been an Epic Fail as a marriage-prep program, given (a) the number of divorces and (b) the number of couples who end up non-practicing Catholics. That doesn't make any particular Confirmation preparation program better or worse, but it does suggest that brevity is not the sole useful criterion.
The Sacrament of Confirmation has no absolute requirements for the recipient other than his being Baptised first, but Matrimony doesn't "work" unless the parties have fully assimilated the nature of Christian marriage and given their full consent. Maybe better outcomes all around would result if all children were confirmed as infants, while the resources that go into Confirmation preparation went to marriage preparation instead.
One of the more important things we studied during those two years was Scripture. The kids, for the most part, didn't come from families that read the Bible on any sore of regular basis, if at all. The first year, we studied the Old Testament, and the prophecies of a Messiah, and the second year, the New Testament and the fulfillment of those prophecies. I think it was pretty important, and was probably the first serious attempt at Bible studies in their lives.
Yes, it was two years, but it was only twice a month, from October to March, and frankly, I don't think that's too much to ask when we're talking about teaching teenagers how to approach their Faith as adults.