They're thinking about something else. They're not in the habit of listening to their speed. They're not in the habit of playing at a controlled speed. How to Deal with the Problem Start early on to make students aware of the regularity of the beat. At the very first lessons and with the very first songs, instill an awareness of speed and pulse .
Metronome use is a second key technique.
As students continue lessons, continue to use the metronome and to tap at lessons as students play. Do not think that just because the student is playing Clementi sonatinas that she doesn't need to hear that constant, immutable tap. Fourth, as the piece begins to take shape, establish a goal metronome speed. All practice should be done below that speed. Discussing speed and setting goals is another way to bring it to a student's attention.