This phenomenological study explored and described the lived experiences of high school biology teachers from a school district in one of the states in the USA concerning the use of online platforms in online biology teaching and learning during the COVID-19 pandemic. The study interviewed teachers to evaluate the teacher experiences, challenges, and opportunities that online platforms presented in biology instruction during the COVID-19 pandemic. The study also analyzed students' perspectives based on the teachers' responses and district data obtained from the student engagement survey administered to all high school students at the school district every year. The findings of the study indicate that students experienced issues such as lack of engagement, unsuitable home environment to support learning, and poor attendance due to minimal monitoring when learning shifted to fully online. Teacher-related factors included inadequate preparedness to use technology to enhance teaching, limited content delivery, and increased teacher collaboration. In conclusion, the study recommends that school districts sufficiently prepare teachers to improve adaptability to different teaching and learning models, emphasizing the use of diverse educational technologies. Future studies should conduct quantitative or mixed studies to establish the extent and degree to which such factors as poor learner engagement contributed to less than satisfactory outcomes in summative and formative assessments. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]