BioEYES, a nonprofit outreach program using zebrafish to excite and educate K-12 students about science and how to think and act like scientists, has been integrated into hundreds of under-resourced schools since 2002. During the week-long experiments, students raise zebrafish embryos to learn principles of development and genetics. We have analyzed 19,463 participating students' pre- and post-tests within the program to examine their learning growth and attitude changes towards science. We found that at all grade levels, BioEYES effectively increased students' content knowledge and produced favorable shifts in students' attitudes about science. These outcomes were especially pronounced in younger students. Having served over 100,000 students, we find that our method for providing student-centered experiences and developing long-term partnerships with teachers is essential for the growth and sustainability of outreach and school collaborations.