Though your first person narrator is telling the story, she’s not writing (or texting) it. As a result, the choices a pre-teen might make in a text aren’t necessary in the narration. In order to create the voice of a pre-teen living in text-culture, listen to those voices that exist in real life. What do you notice about their spoken communication? Consider syntax and diction. Pay attention to interactions and what they can reveal. Understanding some generalities can help you create a foundation for an individual and distinct voice.
You can emphasize this aspect of her culture in other ways. You might show her texting often. What does she choose to text and what does she save for face-to-face interactions? Consider, too, how she thinks about what she texts. She might text one thing but think something different and that discrepancy can be revealing. How she interacts with her phone can also say so much. She might, for instance, check it every few seconds after texting her friend with a question that makes her feel vulnerable. If there’s a particularly important text, you might write it out, but you don’t need to do it for every text.
You’ll help the reader understand your narrator so much more if you focus on authenticity of voice and characterization than on spelling alterations.