I disagree, Rouge, term-time holidays are generally not acceptable! However, practice for a competition is less objectionable.

How about:

Dear *teacher*.

DS1 and DS2 have qualified to attend the state championships for skiing on *date*. As a result I am taking them to *place* to improve their skiing, in order to give them the best chance of sucess/to proceed to represent the state. Please can you inform me of the work DS will be missing during his absence.

Yours faithfully,
Onyx.

That sounds not only important, but like you actually will keep up with the schoolwork too. Not that you have to, but it shows willing.