ah can you post your start.command here then
Loading libraries, please wait...
[17:59:25 INFO]: Starting minecraft server version 1.7.10
[17:59:25 INFO]: Loading properties
[17:59:25 INFO]: Default game type: SURVIVAL
[17:59:25 INFO]: Generating keypair
[17:59:25 INFO]: Starting Minecraft server on *:25565
[17:59:25 INFO]: This server is running CraftBukkit version git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks (MC: 1.7.10) (Implementing API version 1.7.10-R0.1-SNAPSHOT)
[17:59:26 ERROR]: Could not load 'plugins/Korra.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/projectkorra/ProjectKorra/ProjectKorra : Unsupported major.minor version 51.0
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:131) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:328) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:251) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at org.bukkit.craftbukkit.v1_7_R4.CraftServer.loadPlugins(CraftServer.java:364) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at org.bukkit.craftbukkit.v1_7_R4.CraftServer.<init>(CraftServer.java:326) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at net.minecraft.server.v1_7_R4.PlayerList.<init>(PlayerList.java:68) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at net.minecraft.server.v1_7_R4.DedicatedPlayerList.<init>(SourceFile:14) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at net.minecraft.server.v1_7_R4.DedicatedServer.init(DedicatedServer.java:133) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at net.minecraft.server.v1_7_R4.MinecraftServer.run(MinecraftServer.java:436) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at net.minecraft.server.v1_7_R4.ThreadServerApplication.run(SourceFile:628) [craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
Caused by: java.lang.UnsupportedClassVersionError: com/projectkorra/ProjectKorra/ProjectKorra : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.6.0_65]
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:637) ~[?:1.6.0_65]
at java.lang.ClassLoader.defineClass(ClassLoader.java:621) ~[?:1.6.0_65]
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141) ~[?:1.6.0_65]
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283) ~[?:1.6.0_65]
at java.net.URLClassLoader.access$000(URLClassLoader.java:58) ~[?:1.6.0_65]
at java.net.URLClassLoader$1.run(URLClassLoader.java:197) ~[?:1.6.0_65]
at java.security.AccessController.doPrivileged(Native Method) ~[?:1.6.0_65]
at java.net.URLClassLoader.findClass(URLClassLoader.java:190) ~[?:1.6.0_65]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:77) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:62) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at java.lang.ClassLoader.loadClass(ClassLoader.java:306) ~[?:1.6.0_65]
at java.lang.ClassLoader.loadClass(ClassLoader.java:247) ~[?:1.6.0_65]
at java.lang.Class.forName0(Native Method) ~[?:1.6.0_65]
at java.lang.Class.forName(Class.java:249) ~[?:1.6.0_65]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:40) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:127) ~[craftbukkit.jar:git-Bukkit-1.7.9-R0.2-20-g0b2ed13-b3108jnks]
... 9 more
[17:59:26 INFO]: [LANBroadcaster] Loading LANBroadcaster v1.5
[17:59:26 INFO]: [Essentials] Loading Essentials v2.13.1
[17:59:26 INFO]: [EssentialsChat] Loading EssentialsChat v2.13.1
[17:59:26 INFO]: [EssentialsProtect] Loading EssentialsProtect v2.13.1
[17:59:26 INFO]: [EssentialsSpawn] Loading EssentialsSpawn v2.13.1
[17:59:26 INFO]: [EssentialsAntiBuild] Loading EssentialsAntiBuild v2.13.1
[17:59:26 INFO]: Preparing level "world"
[17:59:26 INFO]: Preparing start region for level 0 (Seed: 5663722411217111438)
[17:59:27 INFO]: Preparing spawn area: 89%
[17:59:27 INFO]: Preparing start region for level 1 (Seed: -5402087130358618586)
[17:59:28 INFO]: Preparing start region for level 2 (Seed: -5402087130358618586)
[17:59:28 INFO]: [LANBroadcaster] Enabling LANBroadcaster v1.5
[17:59:28 INFO]: [LANBroadcaster] Broadcasting server with port 25565 over LAN.
[17:59:28 INFO]: [Essentials] Enabling Essentials v2.13.1
[17:59:29 INFO]: Essentials: Using config file enhanced permissions.
[17:59:29 INFO]: Permissions listed in as player-commands will be given to all users.
[17:59:29 INFO]: [EssentialsChat] Enabling EssentialsChat v2.13.1
[17:59:29 INFO]: [EssentialsProtect] Enabling EssentialsProtect v2.13.1
[17:59:29 INFO]: [EssentialsSpawn] Enabling EssentialsSpawn v2.13.1
[17:59:29 INFO]: [EssentialsAntiBuild] Enabling EssentialsAntiBuild v2.13.1
[17:59:29 INFO]: Server permissions file permissions.yml is empty, ignoring it