Are those sentences grammatically complete?

Use of cell phones in classroom for non-school related activities during class time by playing games when they have to listen to what their teacher is saying. Students do not listen to what their teacher is saying about their work; they are busy playing games, texting, on internet or listening to music. That’s why they get bad grades.

No.

These would be proper grammar, each with a subject and a verb:

Use of cellphone in class for non-school related activities is prohibited.
Students using cellphones in class and playing games do not listen to what the teacher is saying.
When students are busy playing games, texting, on the internet or listening to music during class, they are not paying attention to what the teacher is saying, and they get bad grades.