They coat the 7th Year Girls’ panties in magical Itching Powder

(Fred & George dropped out of Hogwarts, and in this reality, they are able to come back the next year to finish. Harry, Hermoine, and Ron's 7th year class)

Fred and George know that itching powder has been around for years, but they have a better idea. They bought a load of Muggle itching powder and used magic to make it better. This enhanced powder would cause a woman's pussy to itch like mad, and only cum could make it better.

Fred and George slip into the Hogwarts laundry and treat the crotch of every 7th year's girls panties with the powder.

They then slip out.

The following morning, the laundry is delivered.