The title is "13 Frightened Girls!" but when counting them (multiple times) there are clearly 15.
When Candy's father shot into the limousine door lock to break Candy out and save her from the bomb, she crouched down in the back seat and hid directly behind the door lock so that he was shooting directly at her head from only a couple of feet away, instead of moving to the far side of the back seat to get away from the door. At that range, his shot should have gone through the door and hit her in the head.
The long shots of the school bus on the mountain road, both in the early scenes of it leaving the school and the later ones of its returning, show only two or three people in the bus, even though in the interior shots the bus is crowded with all the girls.
In the opening voice over by Candy, she states that she and her classmates are the daughters of diplomats. Mai-Ling is not the daughter but rather niece of one.
At the Chinese Embassy, what is spoken is Cantonese. It is more likely that those with the Chinese diplomatic corps would instead speak Mandarin.
In Belgravia, London there are American post boxes and street lamps.
Kathy uses her friendship with the 13 girls of the boarding school to infiltrate herself in the Embassies. None of the 13 girls is from Spain, but in one scene Kathy enters in the Spanish Embassy to do one of her missions.