First things first, have you ever talked to her in person? If not, put down the phone / computer and go do that before you bother a total stranger with strange messages. You don’t want this to happen:
You: Hey what do you think about Mr. DumbTeacher?
Her: Who is this?
You: This is Bill from 3rd period.
Her: Eww you total stalker freak! Don’t message me again!
If you have talked to her, make sure you’ve done so recently. Again, if you haven’t, put down your magical communication device and go do that.
Good? Now. Here’s the magic trick. When you message her, make it about what you last talked about. There you go, you have successfully created a new channel of communication with a beautiful lady and that is hopefully going to turn into a fun relationship.
Run out of things to talk about? Wait until you see them again. Don’t blow up their phone just because OMG she’s finally talking to me I need to drown her in texts so she knows I care about her. Keep it light.
You don’t start conversations with strangers online. You continue them. Follow that rule and you should never be called a stalker creep.