Pandora, Spotify, Slacker, DI.fm, and iHeartRadio are great websites where you can stream music at school. Pandora is a free music recommendation radio service with an endless variety of radio channels. Slacker and iHeartRadio are free services that offer a variety of music radio stations and channels.
Spotify is a free service that allows you to access individual songs from a large number of artists, but the service does require you to register and sign in. DI.fm is a free music service where you can stream over 85 channels of electronic music. Some school computers block certain websites and some of these websites may be inaccessible. Blocked websites vary from school to school. You can get around website restrictions with defilter websites and using software like UltraSurf.