That's not too soon at all. Strike while the iron's hot.
Call her. That way you don't have to waste precious mental resources thinking of what to text, and you also have her invest more of her attention towards you. When she's talking on the phone, she can concentrate only on you, where if you texted, you would be only a minor synapse in her brain and then somethine else in the distraction-laden world of a teenage girl would capture her attention.
Here's the steps:
1. Figure out what you're planning on doing this weekend. Make sure that you're going to do it WITH or WITHOUT her. Example: I'm going salsa dancing on Friday, you're welcome to come along (I actually am, and I invited a girl earlier this week to do so).
2. CALL her. This is VITAL, for the above reason. Talk about something interesting that happened a few days ago, like how you saw this dog chasing a kid on a bike or something (if you can't do that, you might want to look into creating a more interesting life). Remember that what's mundane to you might be interesting and exciting to her. I always have girls ask me about what I do, a boring topic to me is interesting to her.
3. INVITE her out, making sure that she understands that you're going with or without her. This also takes care of any awkward "What do you want to do?" questions that always come up at your age group. If she says no or is a little wishy-washy, don't get emotional or butthurt. She may already have plans. It gives you an opportunity and topic to talk about next time you see/call her.
4. Go out and have fun!
Lastly, welcome to the forum, if you already got a girl's number on the bus at your age, you're already ahead of most guys.