I am writing in response to Kent's concept of "crowd control".
In my mind, running a classroom of "cooperative learning" is something I learned to do when playing role-playing games (I refer to Dungeons & Dragons, RuneQuest, and the many others I've used over the years).
In these role-playing games, all the participants but one play the part of characters in a story. The other person plays the part of "everything else", describing what their characters see, hear, and experience as a result of what they say their characters are doing. This other person also plays the part of every other person or thing they encounter.
When done badly, these role-playing games resemble internet MUD sites: characters exist mostly to fight nasty monsters and have negligible personailities.
Since childhood, the games I was in charge of instead involved the other people slowly acquiring information. The information would allow their characters to solve puzzles and gain influence in the fantasy setting. The fearless heroes did not do so much conquering, but the characters did more growing.
Similarly, when I am teaching math in a cooperative learning setting there definitely is crowd control. My job exists because this material cannot be learned by reading a book. Students need some guidance when first encountering much of K-12 math. The style of guidance differs with the occasion, but I definitely am being manipulative.
For example, when I last taught arithmetic in different bases, I started the lesson with asking six students to come to the front of the room. I handed out papers with 1, 2, 4, 8, 16, 32 on them and as the rest of the class called out numbers the six students would have to figure out how to hold up their papers above their heads to add up to the given number. Then the students were shown MBA blocks, and worked on some activities pretty much on their own; my role then was to pace the groups and make sure they were not going too quickly for some of their members, or believing they were understanding when they were not. Then I came to the blackboard and lead a discussion of how arithmetic works, doing problems correctly or incorrectly and having the class tell me and each other what was right or wrong and why. I was there to guide what they were focusing on: I knew how to think about the math well, and by observing what they were trying to do could help them past points of confusion.
The setting is different from when the band of fearless heroes slowly discovered that the nobleman visiting the prince was actually an assassin of the grey brotherhood, but the idea is the same: my job is to pace the flow of information to the students so that their experience is interesting and memorable, and so that each bit they learn builds off the last in a structured way.
But the important part of the analogy is yet to come. Anyone who has also lead a role-playing game knows he/she can never plan for what the fearless heroes will decide to do. Similarly, I don't design a lesson plan with "do this then do that then do that", but instead preparing how to help guide an understanding of some things to understanding new things. My students often want to use the manipulatives in a way I have not planned on or seen before. Often an explanation that I thought would make things clear does nothing for them. No problem - I've spent my time preparing how ideas relate and not how the class day should go, so I can adapt to their needs.
I am in charge, but my job is to respond to where they are taking the plot. I provide a setting, and they are writing the story.
I've met people who believe any form of manipulation is bad. I'm sure this teaching philosophy would upset them. But from experience I've seen that students of any age normally can't learn many math topcis (for example-probability) on their own no matter how well the workbook is written. Some information needs to be given to students carefully. For me, this is what teachers are for.