Your idea for using Snapchat as a “sandbox” for teaching digital citizenship is interesting. I think, though, it’s important for teachers and students to know that there really is no “safe” place for making mistakes. Even a Snap is a screenshot or third party app away from being saved on someone else’s device. See http://m.wikihow.com/Reopen-Snapchat-Pictures for just one example. Or do a Google search on “how to save Snapchat photos.”
All web/Internet/app users need to understand that once you post something, the places it can go are no longer under your control and are literally limitless.