Dear student, writing a letter to the principal for a genuine cause such as mentioned is a part of formal letter writing.

The format of the letter shall go this way:-

Principal

(Your School/College Name

Address , Pincode )

Date:-

Subject:- A humble suggestion for offline examination.

Respected sir/mam,

Introduction (Name, Class/Branch)

Body (This is in regards to the annual examination that we are going to have ....)

Conclusion (Henceforth, would kindly request for an offline examination.)

Yours sincerely

Your name (class/branch)

Now, the reasons for offline exam could be what you actually face. It could be due to network issue at your place, and lack of concentration giving exams from home and so on.

Hope this helps.

Have a great day!!