I've observed some good strategies used by PreK and Kindergarten teachers. When they see the start of the wiggles, they begin complimenting the students who are doing the right thing. They say things like; "I like how nice Brady is sitting, criss-cross applesauce on the carpet. Good job! And look how Ella and Mariah have their listening ears on. Such a good job!" Once they have pointed out about 3 students doing the right thing, the rest settle down. Another thing you can try, if you know they have been having to sit for longer than 20 minutes is have everyone stand up. Just say "stand up!" You can have them do a wiggle dance to get the wiggles out, turn in a circle on their spot, and then turn to a partner and say good job! Give your shoulder partner a high five and now, speaking very softly, sit down, eyes on me, let's get going.
Strategies like this have to be repeated and used consistently. If your cooperating teacher doesn't use strategies like these, I'm sure she will appreciate you showing her something new.