Because my textbook is bad

[Naguib]

Hello all,
I have a question since my Japanese school textbook is so poorly organized. The sentence that i'd like help on is:
クラスの間は、けいたいでんわをけしておきましょう。
Okay, that sentence is in the "aida" section but what i need help with is the Te form + whatever okimashou is - in this context. I know, according to my book, that it means "i will shut off my cell phone" but why can't you just say けいたいでんわをけます?
I assume that it is part of some sort of construction that my text book fails to go over or that my teacher never explained. I guess I'm bitter. Any help would be greatly appreciated! Thanks a lot,
hannah

[tanitayou]

The __te form +OKU has 3 meanings:

1) completion of a necessary action by a given time (for Ex"buy a ticket before leaving")

2) completion of a necessary action in readiness for the next use (when you finishing using something, put it in the drawer)

3)Keeping the resultant state as it is (Please live the books as they are because I'll use it tomorrow)

Hoping this can be helpful

[trykyn]

all 3 meanings you just mentioned sum up to:
do the action as preparation to something else

so you do this, as the effect of it will come in handy later.

In your sentence, shutting of the mobile is a preparation as not to get disturbed in class