Letter Writing is an art and you can easily be good at it through practice.
Added below is a letter to the PT Teacher requesting Permission to Skip Swimming Classes Due To Injury. This letter can also be used not only for the case mentioned in the letter. It can also be used for any such related scenarios just by editing the reason for Leave.
The portions marked in blue are information that you need to fill in. Please add appropriate details.
From
(Write your name here),
(your standard),
(Your School Name).
To
The PT Teacher,
(your standard),
(Your School Name).
Dear Sir/Madam,
Since I met with an accident last week I will be unable to attend the Swimming classes for next few days. So, I request you to kindly grant me permission to skip the classes until my injury is healed.
Thank You.
Yours Obediently,
(your signature)
(Date)