What do you call a teacher who loses control over her pupils?

There isn't one single perfect word, as the best term depends on the context and severity of the situation. Here are a few options:

* Ineffective: This is a general term suggesting the teacher isn't achieving desired classroom management.

* Disorganized: This implies a lack of structure leading to chaos.

* Overwhelmed: This suggests the teacher is struggling to cope with the demands of the classroom.

* Out of control (herself): This implies the teacher's own behaviour is contributing to the problem.

* Unprepared: This suggests a lack of planning leading to classroom difficulties.

More informal or negative terms might include:

* Losing it: This is a colloquialism suggesting a temporary loss of control.

* A train wreck: This is a very informal and negative description.

The best word to use will depend on the nuance you want to convey.