There is this anime in which at the first episode at the mc's school, the students would be turned into monsters, because of a girl (using a flute I believe) which was together with a other guy. What's the name of the anime?
Could be Blood: The Last Vampire or its spin-off, Blood+
The girl in question could be the antagonist (Diva) who is an excellent singer and I believe it might be her minion who plays the flute.
Other details: One of the protagonist characters (Saya) appears to be school aged and goes to a school and the anime starts in a school setting.
Story features people turning to monsters at least partly in response to music.
There might be more details on the Blood+ Wiki to see if anything jogs your memory.