Complex sentence: Until recently, there was a Tutkmenabat Turkmen-Turkish High School which was part of the countrywide network of Turkmen-Turkish schools, and one the leading schools in the city.
Split into simpler sentences: Until recently, there was a Tutkmenabat Turkmen-Turkish High School which was part of the countrywide network of Turkmen-Turkish schools. Turkmenabat Turkish High School was one the leading schools in the city.