Modern software engineering practice and training increasingly rely on Open Source Software (OSS). The recent growth in demand for professional software engineers has led to increased contributions to, and usage of, OSS. However, there is limited understanding of the factors affecting how developers, and how new or student developers in particular, decide which OSS projects to contribute to, a process critical to OSS sustainability, access, adoption, and growth. To better understand OSS contributions from the developers of tomorrow, we conducted a four-year longitudinal study with 1,361 students investigating the life cycle of their contributions (from project selection to pull request acceptance). During the study, we also delivered a lightweight intervention to promote the awareness of open source projects for social good (OSS4SG), OSS projects that have positive impacts in other domains. Using both quantitative and qualitative methods, we analyze student experience reports and the pull requests they submit. Compared to general OSS projects, we find significant differences in project selection, student motivation, and increased pull-request acceptance rates for OSS4SG contributions. We also find that our intervention correlates with increased student contributions to OSS4SG. Finally, we analyze correlations of factors such as gender or working with a partner. Our findings may help improve the experience for new developers participating in OSS4SG and the quality of their contributions. We also hope our work helps educators, project leaders, and contributors to build a mutually-beneficial framework for the future growth of OSS4SG.