Using data from the English arm of the Health Behaviour in School-aged Children (HBSC) study, we examined the prevalence of loneliness for school-aged adolescents and how it is linked to social inequalities. The HBSC study collects data from 11-, 13-, and 15-year-olds, and is repeated every four years, allowing the exploration of prevalence rates of loneliness pre COVID-19 pandemic for comparison. We also explored whether loneliness was associated with socio-economic status (SES) and linked to academic attainment and health complaints. The total sample was 14,077 from 156 schools in England. Findings revealed a stable prevalence rate of 8.2% for loneliness from 2006 to 2014. We also found, across all survey years, (1) those aged 15 years were significantly lonelier than younger peers, (2) those who reported lower SES were lonelier than their more well-off peers, and (3) higher loneliness was associated with being ‘”below average” academically and reporting more health complaints. Conclusions: These prevalence data enable researchers, policymakers, and others to make comparisons with prevalence rates during the COVID-19 pandemic to explore whether there have been increases in loneliness among school-aged adolescents. Loneliness was consistently related to social inequalities, suggesting that targeted interventions that include whole systems changes are needed.