This is the 1709th most frequent Japanese word.
たる
"たる" means "to be; to constitute; to serve as" and is often used in literary or formal contexts.
In this sentence, 'たる' is used as a classical form indicating a state or quality, specifically associating 'リーダー' (leader) with the qualities expected of one.
リーダーたる者は責任を持たねばならない。
A leader must take responsibility.
Here, 'たる' connects '教師' (teacher) with the requirements or qualities for the role, presenting a suitable quality.
彼は教師たるにふさわしい人物だ。
He is a person suitable to be a teacher.
In this usage, 'たる' emphasizes the inherent value or quality being described, expressing the worthiness or suitability of the subject.