Morgan, as another poster already said, the police are not harassing you.
Please try and see it in a different way. The most important thing in your life is your daughter.
You are obviously worried about her because of the self harm and the bullying.
School are worried about it too. The school and the police are trying their best to look after your daughter. She is more vulnerable than an average child at the moment and they have recognised this.
They were worried about her enough to have the police check she is ok and take her back to school and to talk to you about your partner and Clare's law.
Please try to understand that the police are on your side, what you see as harassment is actually that they care about your daughter.
Please also be aware that if you leave her with this man again it will (as already mentioned) become social services "harassing" you.
My advice to you is if you don't want social services involved then go to the police and make the Clare's law application. You say your partner already told you, so it doesn't matter if you do or not.
When you get it you can check that the dates and so on match what he told you. If it matches what he has said and it's all in the past then at least he has been honest with you.
If it doesn't match up, then you will at least know about it.
Meanwhile if you can build a good relationship with one of the police women, maybe they can help you get the bullying sorted out.
Please remember the police are here to help your daughter. Work with them and things will get better for her.