What is the ruling on playing football with those who wear shorts that reveal the knee or above it, and does the ruling on going to the mosque and facing unveiled women apply to this situation?

The knee is not considered 'awrah (private parts that must be covered), and it is permissible to play with those who cover their thighs, even if something becomes uncovered upon falling. It is incorrect to equate going to the mosque—which is obligatory—with playing with those who expose their 'awrah. Playing is neither obligatory nor a necessity, and the benefits of sports can be achieved with those who adhere to religious guidelines.