Both excellent questions.
1. "Someone you know" is an expression often used to refer to acquaintances, even friends, when we wish to make an understatement for some reason. However, in its strict sense, it means someone whose face you recognize but have not had enough contact with as to warrant a mutual greeting. Eg: the grocer in your neighborhood you don't buy from, someone you've seen hanging with acquaintances of yours but never stood near or exchanged hellos, etc.
"Acquaintance" is someone you've talked to in the past, usually more than once, a bit or a lot, but aren't in a regular relationship with. You sort of naturally sit together if you meet, and converse.
2. As Aspies: nothing. We do the wrong thing and fail and miss opportunities. We mislead people to think we're not interested, or offend them, confuse them, alienate them, etc. That's what happened to you with that girl. We live with it. We suffer. We come to WrongPlanet and ask intelligent questions and think and learn for next time.
Next time you see her, if you like her, convey some body language message that you'd welcome getting a bit closer in your acquaintance with each other. Something like coming up with a smile to where she is standing, or moving to the free seat next to her at a class, etc.
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So-called white lies are like fake jewelry. Adorn yourself with them if you must, but expect to look cheap to a connoisseur.