Silent is an adjective or a noun. Silence is a verb. It should be Silence your phone. Or you can say Keep your phone to a silent status.

Or... you could ask your students to turn off their cell phones.

After a long period of political and social unrest in our country, setting rules is not that easy, especially at school.  I work in a state school, and if rules are not especifically set by the authorities, there �s nothing this humble teacher can do about it.  It sounds hard, but that �s the way it is.  Some students are sensible enough to turn off their mobiles during class (I set the example because I do that in front of them at the beginning of the class) but some others aren �t, so the only thing you can do is ask them to turn them off, and pray for them to do it!  This is a problem teachers have to face every day, and sometimes if you �re too strict you start having problems with the parents, who can even sue you if they feel you �re harassing their children.  Most of the times nothing happens, but it �s not very comfortable to keep on teaching a student whose parents have legal issues against you, is it?  We never give up, though.  In my case, I �m one of those teachers who believe in limits, and in the end the students appreciate that, but in the meantime... I work much harder than the ones who have decided to take the easiest way and of course I �m not the most popular teacher in the school!!