My university lecturer from 8 years back emailed just last week to ask if she could use an assignment I wrote for a magazine article she's having published. I have no intention of ever publishing that piece myself, so I gave her permission, which she now has in writing.
And that's really the crux. It's not as if you're publishing work that they may need exclusivity over further down the line. But, as @Cloudchaser says, you might want to get some legal advice to be sure, and you could always try the legal Stack Exchange site as a start - they've been very helpful to me in the past.
To be honest, I was pretty flattered that she wanted to publish my work, so I wouldn't dream of going back and laying plagiarism accusations at her door. But, she is giving me credit for the piece in the article.
You definitely need permission. In the case of minors from both the student and the parent and make sure it's in writing. It's their work after all. The WITHOUT names may complicate the situation rather than making it easier because you aren't giving credit to the person who wrote it. Depending on how crucial that is, you could ask if they want to remain anonymous or be given credit for the work by having their name included.
You'll probably find that they're flattered you want to use their work and will give you permission without any fuss.
Did a quick Stack search and found this, it's very helpful: