The purpose of this study was to assess the prevalence and associated factors of dental caries and periodontal diseases among 14–19-year-old schoolchildren with limited access to dental care services. A cross sectional study design was conducted during field visits to seven governmental schools in Al-Khomrah district, South Jeddah, over the period from September 2015 to May 2016. Clinical examinations and administered questionnaires were carried out in mobile dental clinics. The dentists carried out oral examinations using the dental caries index (DMFT), the simplified oral hygiene index (OHI-S), and the community periodontal index for treatment needs (CPITN). Statistical analyses were performed using SPSS 20. A total of 734 schoolchildren were examined. The prevalence of decayed teeth was 79.7% and was significantly higher among boys (88.9%) than girls (69.0%). About 11% of students had missing teeth, with a significantly higher figure among females than males (15.9% versus 7.3%); 19.8% of students had filled teeth. Moreover, a DMFT of seven or more was significantly more prevalent among males (43.3%) than females (26.8%), while the percentage of females with sound teeth was significantly higher than for males (20.4% and 9.6% respectively). The CPITN revealed 0, 1 and 2 scores among 14.6%, 78.2%, and 41.6% respectively. Males had a significantly higher percentage of healthy periodontal condition (23.8%) than females (3.8%). Dental caries prevalence was moderate to high, calculus and gingival bleeding were widespread among schoolchildren, and were more prevalent among students with low socioeconomic status.