Maybe I'm dense, but I don't understand what the problem is. OP volunteered for a project. It's realistic to expect that the volunteers will be able to reach one another to discuss the project, plan meetings, etc. The phone number went to the other volunteers, not to the public at large. Did the project coordinator get the number from the school's database? If so, was it flagged, "Emergencies only -- do not give out" or something similar? If not, then I don't think it's realistic to expect it not to be shared with other volunteers on the project. Before cellphones and email and texting and Skype, phone numbers were freely given without a second thought, unless there were extenuating circumstances.
The number is out there now, so this can't be undone. If you don't want people calling you on that number, perhaps you could call or email the project coordinator and ask that you prefer to be contacted by email, or your work number, or some other method.