Using previous student essays as examples
Good question! 

I ask students individually by e-mail if I can use their essays as examples for future classes. Almost always they are thrilled and happy. Then again, I only ask people who serve as positive examples. But I think if you explained to an ESL student that they have the bones of a good essay and that you would like to use it as a sample essay for future students to work on to help improve, I think they would be similarly pleased.

This would be more problematic if I wanted to use the examples in a textbook, used negative examples, or if I posted  essays publicly on the internet. Then I might want a stronger version of a copyright waiver, such as what your previous school uses.