。
Our school beat Keio in baseball.
Literal
We [of-の] school [topic-は] baseball [in-で] Keio [object-を] defeated.
慶応 here refers to Keio (慶應) University, a famous private university in Tokyo — its baseball team is one of the oldest in Japan, part of the historic Tokyo Big Six Baseball League. で here marks the field/sport of the competition. 負かす is the causative-derived verb 'to defeat' (make lose).