Because of illness, he was forced to withdraw from school.
Literal
Illness [because-のため] he [topic-は] unavoidably dropped-out.
やむを得ず ('unavoidably, reluctantly') is a set phrase from やむを得ない ('there's no alternative'). 退学した means 'dropped out/withdrew from school.' Together with のため, the sentence describes a reluctant decision forced by circumstances.