State schools and colleges in England can now order free period products for students as part of a government scheme to tackle period poverty.
Tampons, pads and menstrual cups will be available for primary and secondary institutions to order if they opt in.
The scheme aims to help prevent children missing school if they don't have access to products at home.
Campaigners have warned that schools could disadvantage their pupils if they do not take up the scheme.
Schools will be able to choose from a range of items using an online system, but can also place orders via email or over the phone.
The products, from supplier phs Group, include single-use and reusable pads, applicator and non-applicator tampons, and menstrual cups.
The government is giving each school a set amount of money to spend on products in 2020 - calculated on the basis that 35% of pupils who menstruate will use them.
They come at a range of prices, so it is up to individual schools to decide how they spend their allocated budgets.
It follows the government's announcement last March that it would fund free period products for secondary school students. The pledge was subsequently extended to primary schools.
Amika George, 20, started the campaign to get free period products into schools when she was 17.
She said schools should talk to students about provision, to break down stigma and to make sure they knew the demand was there to opt into the system.
Different students would need different products, she said. For example, pads for children who cannot use tampons for cultural or religious reasons.
Lynda Erroi, head of year seven at Southam College in Warwickshire, said she often works with students who have "no plan in place for when periods start" or cannot afford products.
"This will reduce the stress for any student who is trying hard to attend school when period products are an issue in their life," she said.
"Staff will also feel more empowered that they are able to request supplies and support a child's needs."
The college previously worked with the Red Box Project, which has provided free period products to schools since 2017.
Co-founder Anna Miles said the government scheme could mean the difference between a child attending and skipping school.
She described it as a "step towards genuine equality".
Children and Families Minister Michelle Donelan said the scheme will mean young people can "go about their daily lives" without having to worry.
The Department for Education website says the rate reflects the fact that not all students will need the products all of the time, and is mirrored in a scheme that is already rolled out in Scotland.
Wales introduced funding for free products in schools from April 2019.
One local authority in Northern Ireland offers free products in public places.
Orders from schools are expected to be delivered within five working days.