Question

I have pain in my leg so I could not go absent letter.

Answer 1

To ,
the principal,
(school)
(Ur location)

Respected principal,
I really very sorry to inform u that I will be not able to attend class for a whole week as I have pain in my leg ....I had consult to a doctor and he had advice to take rest of a week ......so, please grant me leave for a week (write the dates)
yours sincerely
Name
class
roll. no.

Answer 2

sir,

sub- could not attend school due to severe pain in my leg

I give my apologies for not attending school from the past few days, the reason behind it being that I had a severe pain in my legs, as a result it was hard to walk.

thanking you 
"your name"


hope it helped