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.
Although frankly this is a good lesson for a bunch of young hackers to learn.