In Greece, a substantial percentage of adolescents start their sexual activity at an early age, leading to increased chance of exposure to various sex-related risks, such as HIV/AIDS. At the same time, sexual education is quite degraded in the Greek school, while little is known about the Greek family’s role in children’s sexual development. The current study of a stratified national sample of 4,090 second and third grade high-school students, with a mean age of 16.45 years, looked into: (a) students’ main sources of sexual information, (b) level of satisfaction from sexual education in family and in school, (c) knowledge about HIV/AIDS, and (d) attitudes towards people living with HIV. Friends (75%) and the internet (55%) were the most frequently reported sexual information sources, much lower being the mother (34%), father (23%) and school (18%). Gender affects the choice frequency of certain information sources, especially family. Satisfaction from sexual education in family was relatively high (60%), but much lower for school (30%). Students present a rather homogenous picture of moderate knowledge about HIV/AIDS and positive attitudes towards people living with HIV. Mother and the internet, as sexual information sources, are related to better knowledge and more positive attitudes towards HIV/AIDS, while the opposite is observed regarding father and TV. Children’s sexual education in Greece and the role of family and school need further examination and systematic interventions. Adolescents’ sexual education and behaviour remain a demoted area of investigation in Greece and more research and systematic interventions are needed.