Translation of "Indispensible" in French

However faultlessly these tasks were achieved, she never commended: it was a maxim with her that praise is inconsistent with a teacher's dignity, and that blame, in more or less unqualified measure, is indispensible to it.

Quelque parfaits que fussent ces exercices, jamais il ne lui échappait un mot d’éloge : c’était une de ses maximes, que la louange est incompatible avec la dignité du maître, et que le blâme, à tort ou à raison, est indispensable.