I'm going to leave as soon as I meet up with another girl.
Are you leaving one girl to go out with another?
Yes, I'm leaving to go out with another girl and I'm already twenty minutes late.
Just tell them the truth. Playing someone just takes too much energy from both sides. Just make it easier for the both of you.
Why? They'll never know. How would you feel if your were them and I told your the truth?
I'd feel upset but I'd appreciate the honesty. Just because someone won't find out and you won't face the repercussions doesn't mean it's okay.