It's a myth that the knees should never go over the toes - try walking down the stairs and not letting your knees go over your toes - it's impossible! However if it hurts then reduce your range of motion - make the movement smaller and adjust to a level that you can tolerate. There's more than knees in this class your hips will thank you too!
Enjoy!
P.S. Apologies but there's a little interference in the first 10 mins of this class, it won't affect your ability to follow the class and it's rectified 10 mins in.