To make the gun effective is more than proficiency in it's use.
Proficiency includes the facets of using guns in a self defense scenario, it's much more than being a good shot
There's being able to make use of said proficiency under extreme stress for a start.
There are firearms training courses that cover that
Then there's being able to identify a threat of sufficient magnitude in time to find the gun in her bag, try to remember where the safety is while she's panicked, then being able to actually retain her weapon.
You're creating a scenario so there are more steps involved so she has the worst odds, but that doesn't mean she won't be able to get to her firearm. Again it comes down to training. I can just as easily have a scenario that she has a quick access concealed holster and has taken sufficient training(drawing, retention, situational awareness, etc). FYI, many concealed carry firearms don't have external safeties. One major concept of concealed carry weapons is decreasing the steps involved in getting to your weapons(firearms, knives, etc)
Enough police officers are shot with their own gun for me to surmise that a teenager (male or female) is unlikely to have the wherewithal to be effective.
What about all the police officers that were able to get to their firearms? Or those that used their firearms successfully in self-defense scenarios? Don't only focus on the fallen officers who were shot with their own firearms. May they rest in peace
Here's a scenario, petite girl is grabbed from behind, both arms pinned to her side and carried away - how does having a gun help her?
Many self-defense firearm courses incorporate anti-grappling techniques and escapes to both access and retain your firearm. There's another thread that covers it
Another scenario - the athletic college guy is walking quickly directly toward the petite college girl, there's a look of anger and intent on his face and he's clenching fists, so she shoots him. Later it emerges he'd just found out his girlfriend was cheating on him and he was stress walking it off...
Situational awareness is a part of firearms training. You can just as easily make her a black belt in your scenario and she has the same poor judgement and she kicks the guy in the head and knocks him out. It's not a failing of the tools(firearms, martial arts, knives, etc), it was a failure of training.
Note: I'm not saying firearms give you 100% percent immunity from harm, nothing does. But there is much more to firearms training than shooting at a paper target.