When a sentence points back to what you just said, the two lock together. The way to point is a pointing word plus a noun — not the pointing word on its own, and not the whole thing said twice.
Boo
"Phones are on in most lessons. Phones being on in most lessons breaks concentration."
It says the same thing twice, and the second sentence sags under the weight of it.
Try it again with a shorter reach: "This habit breaks concentration."