After being attacked by a distressed mother of an obsessed fan, a girl band must escape their dressing room without being killed.

Could work as a comedy. ?But the designated genre is thriller.

I see a funny contained horror. Is the attack keeping them in the room. Is the attack the story? Because you have written it like two separate events. If it is a continuum try something like.

Trapped by the murderius mother of an obsessed fan a girl band must escape their dressing room to survive.

Hope this helps.

Agreed with DPG.

I think you may need to have the mother hold the band hostage, this way she can take away their mobiles and cut them off from contacting the outside world by any other means.

I also think that this could make for a comedy, crazy I know, but think of the princes like preciousness such divas could be imbued in. In a life or death situation they're like totes OMG LOL attitudes could be funny.
Just saying?