。
She isn't cut out to be a teacher.
Literal
She [topic-は] teacher [for-に] is-not-suited.
向く ('to face, be suited to') in the negative ~ていない here ~てない (casual contraction) expresses 'is not suited (to X).' The に marks the role or task one is or isn't suited for. ~に向いていない frames suitability as a state — natural disposition rather than effort or skill. The contraction ~ない (instead of ~ていない) is universal in casual spoken Japanese; in writing one would more often see 教師に向いていない.