Our teacher does not allow us to use mobile phone in class but most of my class mates do including my friend. How can I politely tell her to stop using it in class as she misses out on important lectures and her grades suffer a lot due to it.
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Be honest with her. Tell her that you are worried about her, and that you know it's none of your business, but ask her if she could maybe turn off her phone in classes she attends with you as a personal favour to you. Sometimes I will do (small) things for friends that are important to them but not to me, even if I think they are "technically" wrong about it. Maybe your friend is the same.
You can't stop her. It is her decision to do so, and chances are very high that she's already aware that she misses out on the lecture's content, and that she decided to do it anyway.
All you can do is tell her your opinion on the matter, and hope for her to realize that it in fact is very important to be attentive during class. If she doesn't, or continues to use phone in class anyway, then there is nothing left that you can do , as it is her own decision.
: except tattletale to the teacher, but you would not do her any favor, and she will not stay your friend for long if you go down that road!