Hello Rawson, it's great for you to join us and very sorry for the way your situation is set up, especially when your real dad prefers drugs over you, that must be so disappointing as well as your step dad controlling what's actually happening at home.
Whether or not your parents (stepdad) and your girlfriend's parents don't get on is immaterial to your relationship with her and should have no impact on wanting to be with her, that's a decision made by the two of you and not by either of the parents.
It's just for example like being in a classroom where you and another friend don't like the teacher, this doesn't mean that the two of you shouldn't be friends and the same applies with you and your girlfriend, it has nothing to do with either parent.
Can you involve your mum, because she is your mother and your stepdad is actually not your father, so perhaps she can buy you another phone that your stepdad doesn't know about, and maybe at night you could take out the sim card and hide it because this decision should be your mum's, just a suggestion?
Another option is to meet your girlfriend after school, if this is possible, maybe it's not, but a thought.
You could also tell them that you want to ring Kids Helpline 1800 55 1800 at night in the privacy of your bedroom and would rather talk to these counsellors as you have some private details that you want to remain private.
These are some ideas and hope you can get back whenever this is possible.