I completely agree with you, World Cup or No World Cup students do get distracted. And no doubt it is the duty of parents to take a closer look on their Childs interest as well as in identifying what's best for them.
As Mr. Ravikumar also said that if as a parent we restrict our kids to watch cricket that will give them more curiosity to watch the match! So, we need to find a better way in between the two.
What we can do is; we can sit along with the kid and help them prepare a time table for study. That will give us also a better idea, where our child stood on a particular day. At the same time, during match we should not prohibit them to watch but may be restrict them to watch only selected matches or only selected part of the interesting matches.