My Take
It would be a mistake to dismiss the HPA as crass attempt to capitalize on the Harry Potter franchise in the name of charitable good. A review of their website and its entry on Wikipedia (http://en.wikipedia.org/wiki/Harry_Potter_Alliance) demonstrate that they are a serious non-profit engaged with significant social issues. Mature middle school students and high school students who are looking for social justice causes are the best fit.
The HPA could form the basis for a wonderful unit in project-based learning, especially for students who were (or are) enraptured by the Harry Potter book series. It would be important that students understand that the HPA is not a Potter fan club or fan fiction site, but is rather based on the themes of justice, sacrifice, human rights, and equity that are present in J.K. Rowlings writing. The only "magic" in this site is that which results from people working together to right a wrong. Students will learn that you can't wave a wand to make things better, but with perseverance and hard work you can make a difference.
Your students may initially wish to affiliate with a local chapter (listed on the site), comprised mostly of high school, university, and community organizations. Internet access means that even if there is not a chapter in your local area, you could talk to one or two in your region. Alternatively, you could add an international element to the project by connecting with any of the dozens of overseas chapters.
It would be important that students engage in more that simple fundraising for HPA but also learn activist skills to raise awareness and support within their local community, including thought leaders from government, business, and non-profits. This will require a significant commitment from an adult supervisor who can facilitate introductions and help students navigate the system.
HPA engages in fundraising for its 501c.3 activities. It also offers merchandise, some of which is clearly aimed at older students or adults, such as a t-shirt promoting Dumbledore's Bass Ass Army. You do not want your students buying and wearing to school anything contrary to student dress regulations.
How I Use It
Students who are interested in human rights issues will find plenty of like-minded people at the Harry Potter Alliance (HPA). Just as Harry and his friends and teachers at Hogwarts battled against the forces of evil personified in Lord Voldemort, the HPA finds wrong-doers in the Muggle world through addressing issues such as human trafficking, illiteracy, climate change, sexism and similar causes. For students who are indignant about wrongdoings in their community, nation, or the world, the HPA is a means for engaging this passion along with their love of the Harry Potter book series.