Have you tried ChildLine? I know it seems cliche and all but they can give you good legal and practical advice. Even if you can't access it at home, try asking at school if you can use a friend's phone, and if you've got a school nurse or counsellor try talking to them. They might just want to bring your parents in and speak to them, but make sure to ask if you can call ChildLine or another support line first so they can advise you. If you think that talking to people about your parents is going to make them abusive towards you, make that clear to your school - do not let them contact your parents about this if you think it's going to go badly.
One important bit of advice - even if you haven't got family to fall back on, make sure that you've got somewhere to go if you need to, even if it's just couchsurfing at your friends' houses. Your parents are verging on abusive now, I'd argue that if they start taking away your methods of communication then they are abusive even if it's not physical, and you really need to make sure you've got a way out if you need one.