How did you take your power back in a disruptive classroom? New HS teacher help

My partner is a first year high school teacher, freshmen history. Previously he did long-term subbing but this year he's making all his lesson plans from scratch and teaching two different classes.

He's struggling with classroom management. The kids are the lowest level (called "college prep" here) and can be very obnoxious. He's sort of the cool/nice teacher, so I think between that and being new they're taking advantage of him.

The two major issues seem to be kids acting up in class (distracting others, talking while he's talking, taking their phones out) and kids always wanting to go to the bathroom (or nurse/guidance) during class. His classroom is overfilled - I think 27 or 28 - and he has no other adult in there.

Needless to say, he's stressed. His plan is to go in and talk to the biggest problem class - to tell them calmly that they are under the class average and therefore he's going to be less lenient. And tell them directly that if he has to warn them more than once about a phone or disruption, it will be an automatic write-up. I think enforcing this will show he's not messing around anymore and hopefully get them to shape up.

**Does anyone have any other suggestions on how they took their power back in a disruptive classroom?

Or any tips about managing the bathroom breaks? He's thinking to have a weekly list at the front where kids can silently sign up, and he'll let 3 go at the beginning of class and 3 toward the end. So he can track which ones are going too much. The other teachers are saying to just "say no" but he feels bad doing that, so trying to come up with a plan.

Thanks in advance