I'm sure people have. Because whether or not it sounds like a joke and whether or not the school treats it like a joke, the law certainly doesn't. So if somebody did want to take it further, they could easily do so.
But teachers are aware of this - they are legally requried to be aware.
What you described is termed 'grooming' and whilst teachers aren't meant to be stoic, emotionless machines; constant favours, special treatment or over kindness certainly fall out of the grey area.
By student bullying, I presume you mean the teacher bullying the student? If so, then yes, a lot of times you'll find that teachers get in trouble for it if they take it too far. Normally they'd stop once they are reprimanded, however if they didn't, they'd certainly be up shit creak.
Now, I'm not a proponent for needless utilisation or abuse of the child protection policy, however I am quite glad it exists and people (teachers and students) should be aware of it.