Complex sentence: In April 2015 he filed a sexual-discrimination lawsuit against Columbia, its trustees, president Lee Bollinger and Sulkowicz's supervising art professor, Jon Kessler, alleging that, in allowing the art project to go ahead, they exposed him to gender-based harassment.
Split into simpler sentences: In April 2015 he filed a sexual-discrimination lawsuit against the university and its trustees, as well as its president, Lee Bollinger, and Sulkowicz's supervising art professor, Jon Kessler. The lawsuit alleges that, in allowing the art project to go ahead, they exposed the accused to gender-based harassment.