Plugin not working

[SEVERE] Could not load 'plugins/ForcedGrammar.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: net/reddchicken/ForcedGrammar/ForcedGrammar : Unsupported major.minor version 51.0
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:184)
	at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:307)
	at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:231)
	at org.bukkit.craftbukkit.v1_5_R3.CraftServer.loadPlugins(CraftServer.java:263)
	at org.bukkit.craftbukkit.v1_5_R3.CraftServer.reload(CraftServer.java:629)
	at org.bukkit.Bukkit.reload(Bukkit.java:185)
	at org.bukkit.command.defaults.ReloadCommand.execute(ReloadCommand.java:25)
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:189)
	at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchCommand(CraftServer.java:547)
	at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchServerCommand(CraftServer.java:536)
	at net.minecraft.server.v1_5_R3.DedicatedServer.an(DedicatedServer.java:260)
	at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:225)
	at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:472)
	at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:404)
	at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
Caused by: java.lang.UnsupportedClassVersionError: net/reddchicken/ForcedGrammar/ForcedGrammar : Unsupported major.minor version 51.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
	at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
	at org.bukkit.plugin.java.PluginClassLoader.findClass0(PluginClassLoader.java:87)
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:53)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:249)
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
	... 14 more