Alexis’s purpose in writing the letter is to persuade the principal that the volunteer program should not be required.
In the letter, Alexis begins stating his concern for the school board’s decision of requiring the volunteer program, and in sentence 2, he presents the thesis statement. After this, he gives a number of arguments to support the idea that the decision should be reconsidered. In this way, Alexis’s main purpose is to persuade the principal to change this determination.