How much english would a japanese 9th grader be able to understand?
 

For my japanese class, we are writing to 3rd year jr high students. I don't want the english portion of my letter to be too hard for the person that reads it because they might hate me and decide to send a really hard japanese letter back to me.

I think as long as you avoid using a lot of slang and colloquiallisms then the English teacher on the other end can help out with any problems the student is having. :D

How about you write it in English and then rewrite it in Japanese? That's how I do it with my pen pal in Japan.
This way you avoid problems. I wrote something to my pen pal in Japanese that turned out pretty offensive (thank you MMM, I finally understood why it was wrong), but thanks to the text in English he understood what I wanted to say.
Same thing for me. Sometimes I don't understand something out of his English because uses the wrong verb or tense, but since I read first the text in Japanese it all makes sense in the end.
This way you can keep your English more truthful and so will be the Japanese text.

You are both trying to learn, so help one another the best you can.

just write in simple sentences with minimum number words in each of them. use simple words. many schools have native english teachers now as assistant language teachers. they could help if needed. most japanese schools do offer a language course in english but that is not very popular yet among students.

Well, I think if you just write normally they're understand, and like chryuop suggested, maybe try putting a Japanese translation on the bottom just in case. If you don't use overly perplexing phraseology then conceivably they can comprehend your missive. That's so cool, though, I wish I had a penpal in Japan. Or a penpal at all. Well, good luck!~