This is a really solid writing prompt. It's specific enough to give you good direction, but it is wide open in a lot of other ways. The first thing to decide is who is writing this letter. Obviously, you are writing the letter, but are you writing as yourself or from the perspective of a character from the book? It might be fun to have Leo write the letter to Stargirl and Stargirl write back to Leo.
It also might be really fun to have Kevin write the letters. He is the one character that knows both Stargirl and Leo quite well. The letter that he writes could be a partial apology letter for how he mistreated both of them at certain points, and then the letter could move into giving each of them advice.
Archie is another good character to choose for the letter's author. He is the neighborhood wise man, and he knows both Leo and Stargirl; therefore, it makes perfect sense for him to write a letter of advice on how both characters should live out their post-high school lives.
Regardless of who writes the letters, you have to decide whether or not the letters are going to encourage each character to continue to be the person that they were over the course of the novel. The alternative is to have the advice to Stargirl be something about trying to fit in better, and have Leo's letter advise him to loosen up a bit more.