Site selection is really important for this drill. You need swift and deeper water to ensure safety and a good teaching experience. Most use a large diameter (6") PVC pipe with both ends capped (so it will float). The pipe can be hung off a low bridge, between trees, or between solid rock anchors. Charlie Walbridge uses two stout canoe paddles and some guide belts to secure his strainer pipe - fast and easy to set-up. The pipe needs to stay up on the surface of the river to be effective. Some fire companies use a pressurized fire hose to practice on - this provides even more realism since the cloth covering isn't smooth. Send students down one at a time and have them try both passive and aggressive techniques.
Once the strainer drill is completed, switch to a foot entrapment drill. Thanks to Ron Ray for this great tip. Replace the strainer pipe with some long climbing webbing. Have the students experience a foot entrapment on the webbing. They can easily escape by straightening out their feet. For the more adventurous, have other students work their way upstream to release the student foot entrapment manually. You can also provide the entrapped student a paddle to lean on. Many find this exercise quite valuable.