Some years ago, I asked the same question in a forum and received the answer that it is in fact a charity.
However, I suppose there may be better charities to donate money to, as that school apparently must use a part of donations to keep their mailing going. The letter I saw a few years ago was sent to a relative of ours in Germany, with letter and info material in German language. That relative donates regularly to animal rescue and a few projects with esoteric leanings, and it seems their address got sold to the school. The letter also came with prepared address labels and some gagdets of which I just remember one, a cross on a chain. The letter also had photos of decoratively poor ndn children, contrasted with photos of happy children in the school. I found it somewhat hard to believe the school was indeed a charity when I saw that letter.
I suppose these mail campaigns use up quite an amount of money, not only sending letters in more than one language plus gadgets, but also for buying addresses. Also, if I remember correctly, the school is run by the Catholic church, and they certainly don't only have the money to finance annual mailing campaigns, but also the money to operate the school properly.