I don't disagree but the answers would be part of the bill, surely.
I doubt they would be.
A law would simply declare what is and is not lawful. It wouldn't provide any answers as to how schools actually deal with this on the ground - which, of course, they are already. To which most school's answer would be 'fine, but how?'
I think it is very unlikely that any sanction for mobile phone use in school would be a legal sanction - it would remain, as it already does, under the school's legitimate behaviour policy. So if a school confiscates a phone, because of unauthorised use under the school's behaviour policy, it would remain just that. A sanction under the school's existing authority - a law saying the use would be unlawful would be pretty well irrelevant as I somehow doubt that the school would ring the police ... and the police would pitch up at the school to arrest the pupil.
What schools need isn't 'musts', but 'shoulds' which are backed up by support and examples of best practice which are likely to work in the relevant school setting (noting that not all schools are the same). Statutory guidance of this nature needs no bill in parliament - it can be decided on and implemented via the relevant department - in this case the Department of Education, as happens every year without any resource to a bill in parliament.