When Evil Reigns (2006)

It was a day like any other, but for a group of school friends it quickly becomes a desperate fight for survival when heavy rain turns those caught in the downpour into homicidal maniacs. Locked inside the school as violence erupts on the streets the group might think they are safe, but the infected are waiting for them just outside the poorly secured doors. And they want in. - C. Villinger IMDB