completely different ⇢ M H
this is something completely different: meaning they don't spend time together by kissing each other's necks - neither being friends - it is something completely different.
When they're alone somewhere, they comfort each other or simply talk about whatever. Instead of a "friends with benefits" relationship, their relationship can be defined by "not friends, just being there for each other"
when they're among other people, such as in school, they completely ignore each other. If they somehow have to talk to each other, it is always with emotionless, cold and uncaring words.
Nobody knows what they have together. Nobody knows what they do together. Everyone simply thinks that they're classmates, meaning that they're nothing more, nothing less.
In social they're complete strangers, whilst alone, there's no secrets to keep. They know everything about each other. They know more about each other, than they know about themselves or somebody else.