Charities say many schools and colleges are not aware they can order free period products through a government scheme, amid concerns Covid-19 has left more pupils struggling to access and afford tampons and pads.
Just under 40% of state schools and colleges have placed orders since it was launched in England in January.
The government says the opt-in scheme has operated throughout the pandemic.
But campaigners say it should be promoted more as schools return.
Gemma Abbott, from the Free Periods group, said "many schools and students [are] still not aware" of the scheme, which aims to help prevent children in colleges, primary and secondary schools missing lessons if they don't have access to products at home.
"The government also needs to take some responsibility for the fact that more than 60% of eligible schools and colleges have yet to sign up to the scheme," she said - adding that Free Periods has "hardly heard anything" from the Department for Education (DfE) since the initiative's launch.
"If DfE really is committed to ensuring that 'no young person's education is disrupted by their period', as they said back in January, then they need to make much more effort to ensure that schools and colleges know about the scheme, that they place orders for products and that they distribute those products efficiently and sensitively to students who need them."
She added that the government had suggested that the "opt-in" nature of the scheme would be reconsidered if there was insufficient uptake.
"If these stats have not improved quite drastically by October half-term, this surely needs to be considered," she said.
Institutions that opt in can order from a range of items online, via email or over the phone using an allocated budget for 2020.
The DfE said it expects uptake "to return to pre-lockdown levels" when children go back to lessons - and that some schools did continue to operate the scheme during lockdown.
Benfield School in Newcastle is one of those. It stayed open for keyworker children, but staff delivered products to the homes of pupils who were eligible for free school meals.
Image copyright Sarah Wardle Image caption Benfield School in Newcastle asked its students for their insights on tackling period povertyHowever, assistant head teacher Sarah Wardle said with schools facing so much turmoil, tackling period poverty may not be a priority or a possibility for others. She thinks making it mandatory for schools to use the scheme - and to educate about periods more broadly - would increase uptake.
"It might be that it needs to come to that so that staff sit up and say 'we need to do something about this'," she said, adding she expects demand to rise in September.
"Some of our students' parents might have just been earning enough in terms of them not being able to access free school meals. I think one of the issues when we come back is that those people who were just on the cusp might have lost their jobs, might not be working, and there will be a need."
The scheme is demand-led, so uptake is monitored and used to determine spending.
Image copyright Sarah Wardle Image caption Staff at Benfield School in Newcastle have raised the issue of period poverty at parents' eveningsPhs Group, which supplies the products for the government's scheme and runs the online portal for orders, said it expects uptake to "rise considerably" in September.
Warren Edmondson, managing director of phs Direct, called the scheme is a "major achievement" in tackling period poverty, but added: "The onus is now on schools and colleges to claim their free products and orders must be received by the end of the autumn term."
Amika George, who started the campaign to get free period products into schools as a teenager, said the government needs to do more to promote it, particularly as the pandemic has been a "disaster" for those who were already struggling to afford products.
Image copyright The Pink Protest Image caption Amika George"Household incomes have been squeezed and with so many jobs feeling fragile, children are not even asking their parents for money for pads," she said, adding that young people may have lost part-time jobs to buy their own.
She wants the government to contact schools and offer help if they are not ordering through the scheme.
According to research by charity Plan International, three in 10 girls in the UK have experienced issues either affording or accessing period products during the pandemic, while one in five said their periods have been harder to manage due to the lack of available toilet roll.
The charity's Katie Morrison said that while coronavirus is one factor affecting uptake, some schools may not see period poverty as a relevant issue or priority for their pupils.
"Others may want to engage, but don't know how to go about it," she said.
The charity has released guidance for schools going back in September, which advises on how to deliver free product schemes and to facilitate discussions about what pupils need.