[susan53]

Two possibilities :

1)This would have to be done in a smiley joking way, and only if you think they'd take it OK - depends on their personalities (and yours) and your relationship with them - but I've done it with one class and it worked ...

Wait till it happens again and then ask the person involved to stand up and come out to the front of the class. As they do so, go and sit in their place, with an Oh well ... type of look and gestures. When they look puzzled, invite them to go on with the lesson as they obviously want to be the teacher and you feel quite redundant.

If you do this every time someone makes a correction, they soon get the point.

2) Another possibility : for each activity leave one person out (a different person each time). She acts as the language monitor and must write down anything she hears that she thinks is a mistake, together with the correction. No-one else is allowed to correct because you are "testing" that person's ability to identify mistakes. At the end of the activity she passes the list of things she's overheard to you. You select some of the things which you think it would be worth the class noticing, and focus on them. You can leave out any mistakes that you know come from insecure students and select only those made by students who will appreciate it. You also focus on anything she's miscorrected.

This "channels" the unwanted behaviour into something more positive, and places it back under your control.

Hope that helps.
Sue