On Poppy's first day at lunch, she tells her dorm mates she can't eat the food because she is "pescetarian Monday through Wednesday, fruitarian Thursady through Sunday and vegetarian ALWAYS". This isn't possible as pescetarian's eat fish and vegetarians don't. The two were mixed up.
When Poppy arrives at Abbey Mount she's talking to Kate and says "I choose my friends, and FYI you don't make the cut." Then Kate replies with "I'm sure that comment would be a lot worse if I knew what FYI meant." But as they're walking to lunch Kate is telling Poppy the rules and she clearly says FYI.
During Poppy's date with Freddy when she's driving the car, you can see a strip of blonde hair. But, earlier in the film Poppy and her friends are seen at a hair salon, and she has her hair dyed dark brown. Yet you never see it again at any other time.
When Harriet is taking the "dirty call", it is clear she is using a school phone. However, in the next scene, when we see the fliers in the phonebox, the contact number begins with "07", the digits of a mobile phone number.
When Mrs Kingsley is leaning through the car window talking to Poppy she uses the finger of her left hand because her right hand is on the car when the camera is behind her but when the camera is in front of her she is using a finger on the right hand.
On the website of Abbey Mount school it says it admits boarders from the ages of 11-17. During the credits, the first thing that comes up is a workbook belonging to Poppy and you see that she is supposed to be in Lower Sixth. Since, if they have Lower Sixth they must have an Upper Sixth then Abbey Mount would admit girls from 11-18.
At the beginning of the film, Poppy jumps into the ocean yet when she emerges a few seconds later, she is almost completely dry. Even in Malibu sun this isn't possible. Also Poppy is very concerned when her clothes get wet at the boarding school but this does not bother her when she jumps into the ocean.
In the scene where Kate does Drippy's eyebrows, the matron calls lights out. Much to her roommates' dismay, she gets on her computer and discovers there is no wifi. However, after the lights go out, but before she gets on her computer, you can clearly see the blue light (the light comes from the screen of her computer), as though her computer is already open.