server doesn't work

[hesselino]

when i start my server it goes:

16 : 39 : 52 [sEVERE] unexpected exception

java.lang.unsupportedClassVersionError : net/minecraft/server/mod_IC2AdvancedMachines : Unsupported major . minor version 51.0

than it shows some files and then repeats the process

can someone help me? ???

[havocx42]

install java 7