How would I add a persistent high score?

Solution 1:

PlayerPrefs helps you to store game data.

Usage:

void SaveHighScore(float value)
{
    PlayerPrefs.SetFloat("HighScore", value);
}

void LoadHighScroe()
{
    float hs = PlayerPrefs.GetFloat("HighScore", 0.0f); // 0.0f here is default value when key/value not exest in PlayerPrefs
    ScoreText.text = hs.ToString();
}