The mental hospital substituted as William’s school? Isn’t that the same building they used when that old man wanted to show his dick to Scully and Mulder got jealous in the 1990s?

No I just thought that it looks a bit like William’s school in season 10, and as we know the first episode will by mythological, maybe that’s why they filmed there two days ¯_(ツ)_/¯. Or it’s the hospital Scully or Mulder are in. In both cases, both of them might have been on set!