Ministers risk creating "mask anarchy" unless regulations on face coverings in schools are made clearer, the chair of the Commons Education Select Committee has warned.
Robert Halfon, Conservative MP for Harlow and a former skills minister, insisted that "definitive regulations" must be put in place on whether students should wear face coverings.
Teachers are being put under "enormous pressure" because of the current confusion, he added.
Schools reopening: Masks required in secondary classrooms
Exclusive: Students allowed to break school mask rules
MPs also heard that many teachers are "worried" about a full return of schools, with some opposition MPs pressing the government to adopt a phased return in England.
With schools set to reopen in England on 8 March, only secondary school students are being advised to wear masks when social distancing cannot be maintained.
Covid: 'Definitive rules needed on face masks in schools'
Mr Halfon told the Commons: "Given that the schools minister [Nick Gibb] said that the wearing of masks by pupils on the school estate is advisory guidance, if a pupil or a parent, on behalf of a pupil, objects to comply with the wishes of a head teacher to wear a mask, are we not in danger of creating mask anarchy?
"Enormous pressure is being put on headteachers in Harlow because of the confusion, like Vic Goddard, headteacher of Harlow's Passmores Academy.
"Is it not better to come down firmly on one side or another and provide clearly definitive regulations to help teaching staff?"
Mr Gibb responded: "Well, we said very clearly that we strongly recommend students in secondary schools to wear face coverings in classrooms where it's not possible to keep social distances between pupils.
"And we've also said for quite a number of months that where in communal areas of a school it's not possible to maintain social distance, then staff and adults and students in secondary schools should also wear face masks."
Earlier, Conservative MP Laura Trott told education secretary Gavin Williamson: "Teachers across Sevenoaks and Swanley have done a brilliant job at keeping schools open throughout the pandemic.
"However, many are worried about a full return.
"Will [Mr Williamson] do all he can to share the evidence with headteachers about the low infection risk in schools so they are fully equipped to reassure teachers that schools are safe?"
Mr Williamson said summarised data and evidence had already been published, adding that "extra steps" were being taken to ensure Covid testing at secondary schools in a bid to provide "confidence to the whole education community and wider community".
Labour MP Ian Lavery suggested the government was "ignoring the science" by reopening schools in England in one go rather than adopting a phased approach, adding in the Commons: "With this government forging ahead regardless, how much must be done to tackle the critical issues of PPE [personal protective equipment], adequate ventilation in classrooms, and vaccinating? We really need to protect our key workers."
Mr Gibb replied: "Every step of the way, we've followed the science."
For Labour, shadow education minister Toby Perkins suggested Mr Williamson should "step aside" unless he could provide the required "leadership and ambition" to support a generation of schoolchildren.
Mr Williamson also did not rule out the prospect of the school year being restructured to reduce the length of summer holidays.
Conservative MP Imran Ahmad Khan said: "I wonder if the minister would kindly look into the proposal of restructuring the school year to reduce the length of summer holidays – a policy that would greatly benefit pupils and parents?"
Mr Williamson responded: "We've asked Sir Kevan Collins [the new education recovery commissioner] to look across a whole and broad range of different ways of giving children a boost in terms of being able to not just catch up in terms of any learning that they've lost, but actually, more fundamentally, make major changes to actually how we drive educational attainment over a generation and more.
"And, of course, all of this is something that Sir Kevan will be looking at."