Using previous student essays as examples I teach English as a second language to pre-college adults in the U.S., and I'm interested in the idea of using essays from previous students as examples in a writing class. At a previous institution we used a permission form that students signed giving the university the right to reproduce or modify written work, in part or whole, and with identifying information removed. 

However, as an adjunct who will likely work at multiple schools, I would like to have that permission myself as well. 


What would be the legalities to consider doing such a thing? 
If I have students' written permission, should I still have permission from the institution to do this? 
Will a signed statement (in English), given by someone whose understanding of English is demonstrably weak, function the same as any other? 
Is there a precedent for instructors to gain this permission? As opposed to the institution as a whole. 
Would a blanket statement applying to all assignments work, or should it be for each individual assignment? 


Edited to clarify: I'm interested in both positive and negative examples. Perhaps more so negative ones since the errors produced by international student populations would be more authentic and difficult for me, a native speaker, to reproduce. 

I also have no intention of publishing them outside of classroom materials. Anonymous Mathematician made an excellent point that withdrawal of permission would be impossible if I did this. The only foreseeable publishing I can imagine would be as a course pack or teacher's guide given to the institution or other teachers, but I would still want to ask for additional permissions to do this.
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.