Introduction: Responsible sexual behavior is crucial for adolescents, but lack of knowledge about sexual health can lead to long-term issues such as sexually transmitted infections and teenage pregnancies. In Croatia, data on the onset of sexual activity and condom use are below average. Sexually transmitted infections represent a global issue with over a million new infections daily. Aim of the research: To examine the attitudes, level of knowledge, and sexual behavior of students from two Zagreb schools and analyze differences based on school, gender, and age. Participants and methods: The study was conducted in June and July 2024 with a sample of 292 students aged 15 to 20 years, using an online survey with 45 questions about sexual health and behavior. Data were analyzed using descriptive statistics. The study was anonymous and conducted with ethical committee approval. Results: Most respondents believe that parents should be the primary source of sexual education, although opinions on the quality of education are mixed. Girls rate sexual education more positively than boys. Only 16.4% correctly identify HIV, and girls are better informed about the transmission of diseases through oral sex. While most feel adequately informed about risks, there is a need for improved sexual education. Conclusion: The study reveals that most vocational school students believe that parents should lead sexual education, while the quality of school-based sex education varies. Older students and girls are better informed about sexually transmitted infections and contraception, highlighting the need for improved educational programs to enhance knowledge and responsible behavior among youth.