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 / 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 who's 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 teachers guide given to the institution or other teachers, but I would still want to ask for additional permissions to do this.