Highscore issues

The two scripts:

 using UnityEngine;
 using UnityEngine.SceneManagement;
 using System.Collections.Generic;
 
 public class LevelManager : MonoBehaviour
 {
     // I didn't quite understand what this reference was good for, so I made a Singleton pattern out of it
     // the instance variable is static, so you can now call on the LevelManager without a reference with LevelManager.instance
     #region Singleton Pattern
     /// <summary>
     /// A static instance of the LevelManager. Accessible from anywhere in the scene.
     /// </summary>
     public static LevelManager instance = null;
 
     void Awake()
     {
         if (instance != null && instance != this)
         {
             Destroy(this.gameObject);
         }
         else
         {
             instance = this;
             DontDestroyOnLoad(gameObject);
         }
     }
     #endregion
 
     //this score persists throughout scenes because of the Singleton pattern
     private int score;
     /// <summary>
     /// Get and Set the score with this. When setting, the entered value is ADDED to the current score.
     /// </summary>
     public int Score
     {
         get{return score;}
         set{score += value;}
     }
     /// <summary>
     /// The first level of your game. If there is a menu, this may be 1, otherwise, it's probably 0.
     /// </summary>
     public int firstLevel;
     /// <summary>
     /// The currently loaded scene, set in the start function every level.
     /// </summary>
     private int currentScene;
     //the high score, pretty self explanatory
     private int highScore;
     /// <summary>
     /// The current high score of the player (Read-Only)
     /// </summary>
     public int HighScore
     {
         get { return highScore; }
     }
     /// <summary>
     /// A List of the scores required to advance to the next level. A main menu might change where the first value goes.
     /// </summary>
     public List<int> requiredScore = new List<int>();
 
     // Use this for initialization
     void Start()
     {
         //we get and store the current scene as an int here
         currentScene = GetSceneIndex(SceneManager.GetActiveScene());
 
         //if this is the first level...
         if (currentScene == firstLevel)
         {
             //set the score to 0
             score = 0;
             //get the high score from the save
             highScore = PlayerPrefs.GetInt("HighScore", 0);
         }
     }
     /// <summary>
     /// Returns the current scene index as an integer.
     /// </summary>
     /// <param name="scene"></param>
     /// <returns></returns>
     int GetSceneIndex(Scene scene)
     {
         int sceneIndex = scene.buildIndex;
         return sceneIndex;
     }
 
     // Update is called once per frame 
     void Update()
     {
         //if you want to save your high score every frame, uncomment the next line, although I think it's unnecessary
         SetHighscore(score, false);
 
         CheckForLevelChange();
     }
     /// <summary>
     /// Are the level score requirements fulfilled?
     /// </summary>
     void CheckForLevelChange()
     {
         //is the score equal to the required score for this level?
         if (score >= requiredScore[currentScene])
         {
             //load the scene additively (next scene in build)
             ChangeLevel(currentScene + 1);
         }
     }
     /// <summary>
     /// Load the specified scene and save the high score beforehand.
     /// </summary>
     /// <param name="sceneToBeLoaded"></param>
     public void ChangeLevel(int sceneToBeLoaded)
     {
         //save the high score first (for safety)
         SetHighscore(score, true);
         //load the scene
         SceneManager.LoadScene(sceneToBeLoaded);
     }
     /// <summary>
     /// Set the high score. If save is enabled, saves it to the PlayerPrefs.
     /// </summary>
     /// <param name="currentScore"></param>
     /// <param name="save"></param>
     public void SetHighscore(int currentScore, bool save)
     {
         //if the current score is higher than the previous high score...
         if (currentScore > highScore)
         {
             //set the high score
             highScore = currentScore;
 
             //if it should save the high score
             if (save)
             {
                 //save it
                 PlayerPrefs.SetInt("HighScore", highScore);
             }
         }
     }
 
     //are you sure you want to use the old IMGUI system? The new one is a lot better to use
     //I just left it in for now
     void OnGUI()
     {
         GUI.Label(new Rect(10, 10, 100, 20), "Score: " + score);
     }
     /// <summary>
     /// Has the player died? If so, save the score and load the first level
     /// </summary>
     public void OnDeath()
     {
         //save high score
         SetHighscore(score, true);
         //load the first level again
         ChangeLevel(firstLevel);
     }
 }

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class GUIController : MonoBehaviour
 {
     //a Text showing the current score
     public Text ScoreText;
     //a Text showing the current high score
     public Text HighScoreText;
 
     void Update()
     {
         //set both text components
         ScoreText.text = LevelManager.instance.Score.ToString();
         HighScoreText.text = LevelManager.instance.HighScore.ToString();
     }
 }

I believe this should do it. IF you have more questions, just ask. Or send me an e-mail:

sol.software.soul@gmail.com