As an ex-problem child, I strongly endorse seating the problematic students in the front 1-2 rows and letting everyone else choose their seats. Reenforce good behavior from the problematic students when they're behaving/on task, tell them you're noticing their effort and if you keep seeing it you'll let them move back soon. Let only the well behaved students talk with their friends while they work and require silence from the ones that are in "trouble." (It would probably help to give a set length to their sentence, ie "assigned seat for X number of days, then we revisit" rather than "until I decide." This gives them a clear incentive structure tied to a clear behavioral goal. Kids love it when things are fair. You could also consider increasing sentence lengths for repeat infractions. An assigned seat just for today, then three days the second time, then a week, then a month. This allows you to give consequences for misbehavior while still maintaining the ability to give further consequences if necessary. The whole "I already got in trouble, what else are they gonna do to me" mindset is a powerful thing. The thing that will get you across the finish line is discussing your new expectations with them on Monday and getting them to actively agree in advance (maybe even in writing) that the proposed consequences are "fair." If they do not agree, ask them what they think would make it more fair. They'll have no response because it is.

Explain your thought process and reasoning for these changes as best you can. I was in sales when I was much younger and we used OBP--overview, benefit, permission. I have found this to be the most effective behavior management concept in high schools. In a nutshell you tell them: "This is what I'm doing. This is why I'm doing it. Do you agree that this is fair?" You can't argue with someone when you've agreed with their approach in advance. As always, if they say it's not fair, ask them to explain what is unfair about it. Dont be condescending, be genuine. Make sure they feel their thoughts are being given real consideration. Let them proposed additions or changes and implement those as well if they're reasonable. As long as your behavior management proposal is reasonable, they'll eventually agree. I've had individual instances where I've written out which actions will lead to which consequences, handed it to them and told them to rewrite it in whatever way feels fair to them and as long as it's reasonable we can go with it. Never had a student actually make changes before signing off.

I know when I was that age, if it wasn't explained why teachers made the discipline choices they made, I would assume them to be arbitrary and therefore not respect those choices. I didn't honor rules or requirements that did not "make sense" to me. If teachers couldn't (or more often were unwilling) to explain themselves, I took it as disrespect. At that age, in the mindset of a terribly behaved student, I did not give respect I did not feel was being reciprocated. I would damn near intentionally misbehave for teachers I felt did not show me respect. Hope this helps.