I can't imagine any scenario that justifies an out-of-the blue demand of $50k within a week or your data is deleted. The only way this isn't an awful thing to inflict on a teen education nonprofit is if there have been conversations happening that weren't disclosed in the post - conversations that would have illuminated this possibility.
Although frankly this is a good lesson for a bunch of young hackers to learn.
Not saying this is the case, or that slack thinks it is, but -
If slack found out that the company isn't really a non-profit, or that it violated the requirements in the non-profit agreement (such as promoting discrimination) it would justify a demand for immediate payment in my opinion.
If they determined that Hacker Club violated some terms of the nonprofit demanding they move to regular or be kicked out seems not as bad