There's no single, readily available statistic on the exact number of teens worldwide who finish high school. Data collection on this varies significantly by country, and many countries don't have reliable, comprehensive data, especially in less developed regions. Also, the definition of "high school" and "finishing" can differ internationally.
To get an estimate would require aggregating data from countless national education ministries and organizations, which is a massive undertaking. You would likely find significant discrepancies and uncertainties in the resulting figure.