Is this sentence a complete sentence?

Students do not listen to what their teacher is saying about their work they are busy playing games, texting, on internet or just listening to music, they get bad grades because then they do not know what the teacher has said.

There are three complete sentences:

Students do not listen to what their teacher is saying about their work.
They are busy playing games, texting, on internet or just listening to music.
They then get bad grades because they do not know what the teacher has said.