Complex sentence: In 2009, the school began the construction of a new theatre with over 1000 seats under the basketball courts in the senior school, for the first time, all the senior school will be able to gather in one place.
Split into simpler sentences: In 2009, the school began the construction of a new theatre with over 1000 seats under the basketball courts in the senior school. For the first time, all the senior school will be able to gather in one place.