Hey, I am a clasher and I love playing clash of clans and probably I spend many hours playing it. Let me tell you my personal experience.
I have an elder brother.
Last year he was in 12 standard and got addicted to clash of clans and as all know 12 standard is the class when you have to give your best and bang the best marks you could.
He built his own clan and was doing great in it. But everything in excess is harmull, he clashed day and night and he even followed a technique so that hr never goes offline so no one attacks his base.
And then his board exams where near and he was not focused in studies rather spent time in clashing with other players or spent time on his cell phone. Actually he is good in studies and have a good IQ level and that is also one reason that he got over confident.
Whenever he started studying he was disturbed by the clan mates or coc notification.
My parents use to warn him that this game will not help passing in boards and guided him to study but the addiction was greater than.
So, after board results he got only 75% and was upset about it. But nevertheless he got admission in one of the IP university’s best collage.
But then he never uninstalled coc and never played it again.XD
So, there is no harm in playing coc but remember this game has a great addiction capabilities.