Background: Dental caries and gum disease are a major public health problem predominantly affecting children worldwide. Therefore the present study was conducted to assess oral hygiene practices, prevalence of dental caries and periodontal diseases among 12 and 15 years old high school children in public and government schools of Ambala district, Haryana and to provide data for planning and evaluation of oral health care promotion programs. Material and Methods: A cross-sectional descriptive survey of 12 and 15 years old children in government and private schools was conducted in Ambala, Haryana. A sample of 992 school children was selected by a two-stage cluster sampling method. Dental caries and periodontal health was examined using WHO standard criteria. The student’s t-test and one way ANOVA were used for statistical analysis. Results: The prevalence of dental caries was 34.3% and 46.5% at 12 and 15 years respectively. Mean DMFT in 12 years was 0.82 and in 15 years it was 1.26. More than 90% of subjects in both the age groups were using tooth brush to clean their teeth. Toothpaste was used by 96.7% (228) of subjects belonging to 12 year age group and 93% (703) of subjects who were of 15 years of age. Gingival bleeding was found in majority of subjects. Conclusions: The overall prevalence of dental caries was low but the prevalence of gingivitis was quite high. The observations indicate the need for a school oral health promotion programme to sustain the healthy practices in this growing population