Writing a letter to the admissions office on behalf of my brother: advice?

So my little brother is going to be finishing 8th grade this year and is currently in the process of applying to high schools. His first choice is my alma mater, a prestigious private (Jesuit) high school here in Manhattan – I graduated 12 years ago (yes, there’s a big age difference) and was an average student.

Our concern is that his grades and test scores will almost certainly be below the school’s typical standards. Through 6th grade his grades were simply poor and his standardized test scores quite average; then we started medication for ADD and got a him a (surprisingly) much-needed pair of eyeglasses, and both grades and scores have seen substantial improvement, to the point that his most recent report card is all A’s and B’s, and he was testing up above the 80th percentile last year. Nonetheless, this high school is highly selective, they give a lot of weight to an applicant’s score on the admissions exam for NYC Catholic high schools, and my brother’s scores are likely to be well below their typical standards (despite his recent strides in this area).
On the bright side, the school also places a high premium on tradition, and his being the sibling of an alumnus will work in his favor. To highlight this fact – and to make his case perhaps more eloquently than he was able to in his brief application essay – I intend to write a letter to the school’s Director of Admissions. Since I’m not accustomed to this sort of thing, I was just wondering if anyone here had any advice on form, content, etc. As far as specific questions go, the only one I have is: Is it terribly affected to write via snail-mail instead of email? But mostly I’m looking for any general advice or ideas that I might like.

Need answer (kinda) fast; I’m working on it now, and intend to mail it by Monday at the latest. Thanks.

I would suggest gently acknowledging his weaknesses, but in light of his successes, if you know what I mean. Sort of a “this kid has overcome so much and has so much more room for growth in the right environment, such as your estimable institution, which did so much for me when I was his age.”

Flattery may get you everywhere.

I wouldn’t mention the admission test scores if you don’t actually know what they are. Who knows, maybe he did better than you guess.