However hard it is, however much she begs you and arouses you, just say no. Tell her that you are not going to be a father, that you do not want to see her getting hurt and that you don't want to have to leave her, but if she continues being silly one of those WILL happen. You MUST have self control otherwise you are going down with her. Maybe her mother has said summut to her, or something bad has happend (like she knows that she might already be pregnant?) that is making her act in this carefree and reckless way?
Also, when it comes to what time you need to go home, talk to her mother. Maybe if you seemed concerned and asked what time she would like you to leave the house, she would have a more agreeable attitude towards you. You would also be able to tell your girl that you know her mother doesn't want you here after, say, 8pm.
You need to sit her down and not let her get up/leave until she has told you what is going on. She is being reckless and acting in a stupid and irresponsible manner. She may just be being silly, or there may be summut very wrong.
Also, don't let her guilt you and don't let her fob you off. If she isn't going to take this seriously, you must!