Some people complete deals during school time, I used to because I live in a totally different timezone.
There's a NSFW toggle on HF as well. There isn't really any bad that would come from adding it but extra work, but nonethelesss a decent suggestion.
Just scroll up when your teacher walks by. Your teacher wouldn't do anything besides look at you funny, it's not like it's a teenagers nudes. Don't want to get caught? Get off your phone and talk to people in school rather than scoping the market to scavenge for a "dank" username