Investigators would like to perform a study with the overall goal to analyze the relationship between self-reported online learning due to COVID-19 during the 2021- 2022 school year (students who self-reported online learning should be considered exposed) and depression determined by Patient Health Questionnaire-9 (PHQ-9) score (students with PHQ-9 score > 10 should be considered having the outcome) in adolescents ages 13-17 in Houston ISD schools from the following 2022- 2023 school year. They would like to use single stage simple random sampling . What is the sampling frame in the above study