Slack has raised our charges by $195k per year

Since you're a nonprofit that teaches coding, it could be a great time to consider self-hosting a FOSS chat tool.

Suggestions: Campfire [0] or Zulip [1].

Also, if the data in chat is being held hostage, the org might be using chat wrong. Right tool for right purpose. If starting over, perhaps consider if it would make sense to put that documentation or whatever it is that will get "lost" from Slack into a wiki or repo or other appropriate tool?

Big empathy, though. It must be pretty crushing. But that is why serious geeks have long been for FOSS.

  [0] https://once.com/campfire (recently became FOSS) 
  [1] https://zulip.com

I think it is time we all start moving away from renting software back to owning it (or at the very least, owning a perpetual license). The subscription model is does not exist on a stable plateau. Every company that runs on a subscription model will (and must, by virtue of incentives) to attempt to "develop new revenue streams".