Question When did they go poof?

The full story will eventually come to light but not any time soon. It isn't that details aren't known so much as details aren't being shared with the student body. A decision made, in part because between 'lack of information' and 'enough information to really get the rumor mill going' ... the administration in transition made a conscious choice to try to keep the kids from treating Carson and company's absence as the 'big mystery to solve this school year'. And while there are (behind the scenes) meetings involving the investigations, progress, and so on more or less daily still at this point... its not anything they're going to regularly brief the kids on.

Add into that... the Second Gen Freshman class doesn't even know any of those people. For the most part, they recognize that there's some administrative staff shuffling going on while people get settled into new positions and some late hires happen... but its not like they 'miss' Carson being the Headmistress. They don't have that experience to create any sense of loss or comparison.

As far as getting the dates and other details for the purpose of WhatIFs? Ask your specifics in the Worn Wrench forum and if one of the Gen 2 authors is open to it, you may or may not get details. I suspect if you're just looking for dates, they'll have no problem sharing the data.