It is easy to get a prepaid cell phone for the school, and have a separate number for them to call. But, I think you mentioned another concern as to why you answer the phone, and that is that they may have a question about something that happened during your shift.
I started getting these sorts of calls, mainly the nurse who took my group of patient would call me to clarify something I said during report. Mostly, it was, "I forgot what you told me...." or, "What did you tell me about this patient...."
I figured out pretty quickly that many nurses would rather pick up the phone and call you instead of actually paying attention during report or researching a problem themselves to find the answer.
What I realized is that I am not on-call after I leave the facility. I am not obligated to answer any questions or to clarify anything or especially, to tell a nurse something AGAIN, because she wasn't paying attention during report. What I do to avoid any calls based upon something I may have missed, is to go over my med charting like crazy before I leave. Making sure everything is signed off, especially insulins and IV antibiotics.
But really, my point is, once we clock out, we are off the clock! If you documented your meds properly and gave them as ordered, passed on any new orders to the oncoming nurse, you should not have to answer any questions during the day.
The important thing is to be able to let go of it. We all worry about making a big mistake, but really, once you leave you couldn't fix it anyway. So, we need to let go and just go to bed.
Once they realize you are not going to answer and not return their calls (until a reasonable time), they will stop calling.