We have our own small Pen Pal Forum on our site, but I believe your best choice would be Stefan Schauwecker's 'Japan Guide'. His pen pal forum is organized in gender and age groups, which is quite useful for students. I recommend you hop over there through the link below:
Checking our directory's keyword list I found out that the term 'Japanese pen pals' actually ranks quite high in our users' search queries. I have therefore decided to add a new subcategory just dedicated to Pen Pal related web sites.
You can access it through