A blanket ban on cell phones in schools will hamper teachers and staff too. Plus, the usual reason why jamming is illegal in most countries: dialling emergency numbers!
Get approval to run your own short-range cell site that has a whitelist of IMEI (or IMSI, if that's not available) numbers, and blocks all non-emergency calls made by any phone that's not on the list.
Put the staff on the whitelist, and give parents the option to put their phones on the whitelist when they're visiting.