Write an application to the Principal of your school requesting him to allow you to change your subjects.
To
The Principal,
[School Name]
[Address]
Madam,
I have come to the conclusion that it was a mistake on my part to have taken Commerce with Mathematics as my subject. I am an above-average student and yet I feel I should change them for Physics, Chemistry and Engineering Drawing. My subject teachers have no objection to this change.
I shall be grateful if I am allowed this change as early as possible.
Thinking you,
Yours obediently,
[Your Name]
[Your Class]
[Date]