Why do I keep hearing voices in my head?

This sounds like schizophrenia to me.
You better contact an institution (or better: more than one only) that can help you.
But careful: to get pumped up with 'medication' is not always necessary.
(One of my Exe's had it - it's not funny, but it can be dealt with)
Can't tell you why. Just know you're not alone. I've heard one voice since childhood then a couple years ago that increased. When my mind suddenly broke down the walls inside.
While they say that schizophrenia at your age of 13 is very rare it can and does happen. In my opinion you do have schizophrenia and should seek out medical care before things develop further.
Dissociation. Please, Speak to someone, Even a School Counselor who perhaps can address your parents. It is serious. xx
Opinion
4Opinion
Seems like something you need to check with a mental health professional. It's not a joke, neither is it funny and whoever makes fun of you for such a thing is an idiot. The sooner you go the better.
This sounds a lot like DID if you've heard of it. Its not that uncommon, about the same amount of people experience this as people with ginger hair. I'm no expert, but maybe research DID, there are some useful interviews on youtube etc. (dissociative identity disorder = DID)
Sounds like schizophrenia but I'm no expert. But this is for sure not normal. You should definitely get a diagnosis as soon as possible.
you need medical attention immediately! Not asking questions on a forum.
Ask the voice.
Superb Opinion