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One possible version：
As we all know ，physical exercise after class can help us remove tiredness ，and is helpful to our studies 。however ，some students in our class have been feeling even more tired after physical exercise and often fall asleep in class。
In my opinion ，it isnecessary and important for students to have some exercise ，but we should choose proper sport events and avoid taking part in any extreme or even dangerous events such as mountain-climbing after class and on weekends 。besides ，physical exercise shouldn’t last too long。Half an hour to one hour is ok，I think 。otherwise ，it will make one even more tired after having an extreme sport event for a long time 。do you think so？
Looking forward to your advice 。
Yours faithfully ，
Lihua
略