Alpha is intended as initial evangelization for those who have never heard the initial proclamation of the Gospel. There is an Alpha for Catholics, which takes the presentation of the basic elements of the Apostles Creed a step further and explains the Mass and Sacraments.The next logical step is for this person to enter RCIA and complete preparation and formation for initiation into the Catholic Church.
There is an Alpha tailored for high school youth, for young adults and college students, even for prisons. As a program for baptized but uncatechized (read clueless) Catholic youth this could be a good starting point, but is not really the way to go with youth who are already confirmed or preparing for confirmation. It is not adequate on its own for confirmation preparation, or for post-confirmation faith formation.
We have had an active Alpha program in this parish for 5-6 years. The plan called for all parishioners to attend, and from them to form teams to run the succeeding programs. the year they did not have full-time DRE a volunteer ran Alpha (with a good team) for junior and senior high school students. It went pretty well, because a lot of them truly had not been catechized since first communion, but they also had a further confirmation program for that group.
Alpha also does not replace youth ministry, or parish youth group. It is great for its intended purpose, initial presentation of basic Christian doctrine to those who have never been exposed to it before.