Yes, of course it would be ok to send her an e-mail.
Friends exchange e-mails all the time.
Ask her how school is going and if she has done anything interesting lately?
Tell her what you have been up to, and how you feel about school, etc.
Ask her about her favourite music, or films or sports ~ that sort of thing.
Talk about things that you remember about your old school ~ ask her if the same teachers are there, etc.
