Your friend should understand that chating and hving fun in life can be done whole life if your friend waste this time which precious and very important for his future. if he missed today study so in future he will be sitting and thinking  that if i had not chated in whole my school life so i must be not sitting here. this can be suggestion .
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Will advise him by saying that--
1.fb is addictive so be aware while chatting with frnds
2. frnds are there in schools so what is the need of chatting with them specially in study hours??
3.assignment is more imp. than ur chatting
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