If u r a good ,obidient, honest, truthful student you should never tell a lie and try to escape from that situation instead apologize to u r mistake . if a student is really concentrated and dicipline you will never sleep in the class. So try to be diciplinary and concentrated student in the class, if u r doing so u will never get sleep in the class even if u try.
The major reason for sleeping in class is lack of sleep at night. first of all children should have atlest 8 hrs. sleep.  So try to manage u r timtable according to it. 
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nice ans.
thanks a lot. for u r compliment.
so i punish by teacher than i tell sorry to the teacher