Lavender Brown (Jessie Cave) in Harry Potter and the Half-Blood Prince.
Jessie Cave - As Lavender Brown -- World of Harry Potter list
puberty does strange things to the harry potter cast.: I was very mad when Lavender Brown was re-cast just so Ron could date a white girl.
Ron Weasley and Lavender Brown (Rupert Grint and Jessica Cave) ~ Harry Potter and the Half Blood Prince
The one about Professor McGonagall not being the headmaster isn't true if you read Harry Potter and the Cursed Child