It works but it has to be the exact tag. Example Vanilla stockings -glasses
https://www.fakku.net/search/Vanilla%20stockings%20-glasses
the controversial tags bork the system so you can't search for loli, shota, incest, forced or monstergirl.
In our case you just can't exclude without a include so this should help hentai -school outfit or vanilla -school outfit
https://www.fakku.net/search/hentai%20-school%20outfit
https://www.fakku.net/search/vanilla%20-school%20outfit