Help with "-chan" suffix
 

Hello,
I am studying abroad in Japan. Today, my class visited an all girls highschool & interacted with the students there. Each of us were assigned one girl in particular to introduce ourselves to and be guided by. Miku is 14 and a (third year, I believe) middle school student. I am a 19 year old college sophomore. I want to send her an email. I am wondering how I should address her. She is quite a bit younger than me, thus -san sounds slightly awkward to me. "-chan" seems more natural (given her age and manner) but seeing as we just met today, I don't want to sound overly familiar. Basically, my questions are: does -san sound too formal for such a young girl (addressed by myself, her elder) and if so, is -chan a suitable substitute?
Thanks,
Megan

It all depends on how close the two of you had become to each other by the end of the day. It cannot and shouldn't be determined by the age difference alone.

How were you guys addressing each other at the end of the day? That's exactly how you are going to address each other in your first letters.

Here's a good reference chart that might be helpful:

How to use ~san, ~kun and ~chan in Japanese

I agree with Sashimister that "chan (～ちゃん)" is generally associated with closeness as well as age.

You don't necessarily need to use any suffix. Just using "Miku" is not all that unusual especially if you are just sending email.

Maybe when you get closer she may volunteer a nickname like ミ～ちゃん that friends call her.