Pygame text rendering

I'm learning pygame and followed a tutorial about a pong game.
Everything is from the tutorial except the score system, I implemented.
It does work, but instead that the score is updated, the new score is printed over the previous score.. Which isn't a nice view. How can I solve this?

(I think only the main function is important for this problem, but I'm giving whole the code to be sure)

#!/usr/bin/python

try:
	import sys
	import random
	import math
	import os
	import getopt
	import pygame
	from socket import *
	from pygame.locals import *
except ImportError, err:
	print "couldn't load module. %s" % (err)
	sys.exit(2)

def load_png(name):
	"""Load image and return image object"""
	fullname = os.path.join('data',name)
	try:
		image=pygame.image.load(fullname)
		if image.get_alpha() is None:
			image = image.convert()
		else:
			image = image.convert_alpha()
	except pygame.error, message:
		print 'Cannot load image:', fullname
		raise SystemExit,message
	return image, image.get_rect()

class Ball(pygame.sprite.Sprite):
	def __init__(self,(xy),vector):
		pygame.sprite.Sprite.__init__(self)
		self.image, self.rect = load_png("ball.png")
		screen = pygame.display.get_surface()
		self.area = screen.get_rect()
		self.vector = vector
		self.hit = 0
	
	def update(self):
		newpos = self.calcnewpos(self.rect,self.vector)
		self.rect = newpos
		(angle,z) = self.vector
		
		if not self.area.contains(newpos):
			tl = not self.area.collidepoint(newpos.topleft)
			tr = not self.area.collidepoint(newpos.topright)
			bl = not self.area.collidepoint(newpos.bottomleft)
			br = not self.area.collidepoint(newpos.bottomright)
			if tr and tl or (br and bl):
				angle = -angle
			if tl and bl:
				angle = math.pi - angle
				player2.score+=1
			if tr and br:
				angle = math.pi - angle
				player1.score+=1
		else:
			player1.rect.inflate(-3,-3)
			player2.rect.inflate(-3,-3)
			if self.rect.colliderect(player1.rect) == 1 and not self.hit:
				angle = math.pi - angle
				self.hit = not self.hit
			elif self.rect.colliderect(player2.rect) == 1 and not self.hit:
				angle = math.pi - angle
				self.hit = not self.hit
			elif self.hit:
				self.hit = not self.hit
		self.vector = (angle,z)
	
	def calcnewpos(self,rect,vector):
		(angle,z)=vector
		(dx,dy)=(z*math.cos(angle),z*math.sin(angle))
		return rect.move(dx,dy)

class Bat(pygame.sprite.Sprite):
	def __init__(self, side):
		pygame.sprite.Sprite.__init__(self)
		self.image,self.rect = load_png('bat.png')
		screen = pygame.display.get_surface()
		self.area = screen.get_rect()
		self.side=side
		self.speed=10
		self.score=0
		self.state="still"
		self.reinit()
	
	def reinit(self):
		self.state="still"
		self.movepos=[0,0]
		if self.side=="left":
			self.rect.midleft=self.area.midleft
		elif self.side == "right":
			self.rect.midright = self.area.midright
	
	def update(self):
		newpos = self.rect.move(self.movepos)
		if self.area.contains(newpos):
			self.rect=newpos
		pygame.event.pump()
	
	def moveup(self):
		self.movepos[1] = self.movepos[1] - (self.speed)
		self.state = "moveup"
	
	def movedown(self):
		self.movepos[1] = self.movepos[1] + (self.speed)
		self.state = "movedown"

def main():
	pygame.init()
	screen = pygame.display.set_mode((640,480))
	pygame.display.set_caption('Basic Pong')
	background = pygame.Surface(screen.get_size())
	background = background.convert()
	background.fill((255,255,255))
	global player1
	global player2
	player1 = Bat("left")
	player2 = Bat("right")
	speed=13
	rand = ((0.1 * (random.randint(5,8))))
	ball = Ball((0,0),(0.47,speed))
	playersprites = pygame.sprite.RenderPlain((player1,player2))
	ballsprite = pygame.sprite.RenderPlain(ball)
	screen.blit(background, (0,0))
	pygame.display.flip()
	font=pygame.font.Font(None,50)
	clock = pygame.time.Clock()
	while 1:
		clock.tick(60)
		for event in pygame.event.get():
			if event.type == QUIT:
				return
			elif event.type == KEYDOWN:
				if event.key == K_a:
					player1.moveup()
				if event.key == K_z:
					player1.movedown()
				if event.key == K_UP:
					player2.moveup()
				if event.key == K_DOWN:
					player2.movedown()
			elif event.type == KEYUP:
				if event.key == K_a or event.key == K_z:
					player1.movepos = [0,0]
					player1.state="still"
				if event.key == K_UP or event.key == K_DOWN:
					player2.movepos = [0,0]
					player2.state="still"
		text=font.render(str(player1.score)+" | "+str(player2.score),1,(0,0,0))
		screen.blit(text, (320,10))
		screen.blit(background, ball.rect,ball.rect)
		screen.blit(background, player1.rect, player1.rect)
		screen.blit(background, player2.rect, player2.rect)
		ballsprite.update()
		playersprites.update()
		ballsprite.draw(screen)
		playersprites.draw(screen)
		pygame.display.flip()

if __name__ == '__main__': main()