Why is my level system not working properly?

I'm pretty new to scripting so sorry if it's something that should be obvious. I am trying to make a level system in which the longer players play the higher the level they become. The only issue is that if there are multiple players in the server at once it counts everyone's XP and when the whole server reaches the lowest XP needed for a player to level up it levels up that player.

minutes = 0
xpRate = 5 
function onPlayerEntered(newPlayer)
    wait(.5)
    local stats = Instance.new("IntValue")
    stats.Name = "leaderstats"

    local score = Instance.new("IntValue")

    score.Name = "Money"
    score.Value = 0

    score.Parent = stats    
    stats.Parent = newPlayer

    local level = Instance.new("IntValue")

    level.Name = "Level"
    level.Value = 1

    level.Parent = stats    
    stats.Parent = newPlayer

    local XP = Instance.new("IntValue")

    XP.Name = "XP"
    XP.Value = 0

    XP.Parent = stats   
    stats.Parent = newPlayer

    while minutes <= xpRate do
        wait(60)
        minutes = minutes + 1
        score.Value = score.Value + 25
        XP.Value = XP.Value + 5

    if minutes == xpRate then
        minutes = 0
        xpRate = xpRate + 2
        XP.Value = 0
        level.Value = level.Value + 1
    end
    end
end

game.Players.ChildAdded:connect(onPlayerEntered)