Are these complete sentences and are they grammatically correct?

First, cell phones can distract students when they are concentrating on school work. They can interrupt class time by ringing or receiving a text message when teacher is explaining their homework. When cell phones ring during class they interrupt not only one person but everybody in class, and until the teacher gets everything in order most of their time is wasted. Cell phones can be used for non-school related activities during class time only by emergency situations.

1) "Cell phones can distract students when they are concentrating on school work." Sounds OK.

2) It doesn't make any difference what the teacher is doing, a cell phone ringing interrupts the class. I would say, "They can interrupt class time by ringing or receiving a text message during classes."

3) When cell phones ring during class they interrupt everybody, wasting the time it takes the teacher to restore order.

4) Use of Cell phones in the classroom for non-school related activities is limited to emergencies.