Generative artificial intelligence (GenAI) is penetrating in various social sectors, motivating a strong need for teaching AI literacy in younger generations. While substantial efforts have been made to teach AI literacy and to use AI to facilitate learning, few studies have provided empirical accounts of students' nuanced processes of using GenAI for learning. In this study, we engaged a group of high school students in leveraging ChatGPT to support their knowledge building efforts. Following the teacher's pedagogical design, students used ChatGPT for a range of distinct purposes. Student interviews showed detailed processes of using ChatGPT for knowledge building and students' emerging AI literacy in multiple dimensions. This study offers practical implications for the integration of GenAI in K-12 education and urges educators to create spaces and scaffolds for students to mindfully engage with GenAI in the classroom.