The Cadet Corps train and educate cadets. Only the top 10 graduates can apply for Military Police after graduation. We follow Eren and his friends as they graduate from the 104th Cadet Corps. Unsurprisingly, the 3 Marley "spies", Annie, Reiner and Bertolt occupied the top 3 spots, likely due to their previous military trainings. Or it could also be due to their power of the Titan, as Eren ranked right below them.