There are several ways to accomplish this. Here are a couple of scenarios:
Your lab has a computer at each keyboard station: If your lab has a computer at each keyboard station, you can run an audio cable from the computer's audio output to the audio input of your digital keyboard. When setting it up this way, the student will hear audio from both the computer and the keyboard through headphones. Routing your computer audio through your keyboard speakers
Students bring their own laptop/tablet/phone/etc. to class with them. If students are using their own device to access eNovativePiano during class, they can wear earbuds from their personal device under their keyboard headphones. Gamers do this all the time, as shown here:
Alternatively, you could just connect an audio cable from the audio input of your digital keyboard and simply leave the other end available for students to plug their device in.