I never used Snapchat. I remember when it came out that the main feature was it erased texts after they were read.
Users didn’t have to worry about texts being read by parents or subpoenaed by the cops because they were zapped automatically after reading.
I always assumed that was primarily for illegal activity like drug deals. I was never interested in using Snapchat.
Why are these Cornell 7 texts circulating in the media?
I have not forensically examined Snapchat myself, so take this with a grain of salt, but unless it is known to be a secure app there is nothing stopping you from saving a screenshot to a file.
I would think a screenshot would be very easy to fake and would therefore have limited value as evidence.
I found this help answer.
They delete by default unless the user changes the account settings.
There is also a save option.
It doesn’t sound like there’s much advantage in using Snapchat instead of standard Google Messages.
Apparently Snaps and Chats are two different things. I’m way too old to use this App. 
The competing messaging platform WhatsApp makes much of how the messages are encrypted end-to-end so there’s nothing subpoena-able on their servers; only gibberish. Only you and your fellow members of any given conversation can decrypt incoming messages into the plaintext version.
By default the end user app is configured to auto delete each message when it hits 48 hours old. So by default there’s a short history available, not an indefinite one.
But …
Yeah, there is a setting to disable deleting and then the history builds up effectively forever. And you can copy the textual content and paste it anywhere. And if you receive a photo, that can be auto-saved to your device’s photo collection where it becomes out of reach of Whatsapp’s secure garden. And you can screenshot. And …
So kinda secure for the routine social embarrassments of teens. If they don’t diddle too many of their settings away from the defaults Not a secure way to commit crimes, run statecraft, etc.
Snapchat? Different in detail, but has the same inherent issues.
Googling, someone took a photo of the Snapchat messages using another phone or other device and that had they just used the screenshot function on the phone itself, a notification would have been sent to the participants.
As @OldGuy points out, that hardly makes for convincing evidence. Maybe you could establish it was an unaltered photograph and when it was taken; that might be something.
I don’t think anyone has presented the photos of the conversation in a court of law yet, so that may not be relevant. Are any of the frat boys in question denying the contents of the chat?