I've been a teacher for decades and known quite a few girls this has happened to - and I haven't judged them in the slightest! It's not unusual to be sexually active at 15, and though personally I would advise to wait a bit longer in an ideal world, I certainly don't judge or blame young women who don't in any way!
I do potentially judged their partners, depending how old they are - so once you have sorted your current situation, it might be worth reflecting on how you feel about the sex you are having, do you feel pressured into it, is it something you want to keep doing? It's really common for young people hour age to be pressured into sex by older partners, and there is definitely support out there for you if you feel in any way that may be happening.
But right now, you need to decide what to do about the pregnancy. I would strongly recommend termination in your circumstances personally, as it does not sound like you would have much support in raining a child. (Though ultimately it's your body and your choice).
Can you phone your GP surgery Monday morning and make an appointment? Is it somewhere you can get to on foot or on a bus? If you show school evidence of your appointment, you should be able to sign out and not get into trouble for truanting.
And it would be much better, practically and mental health wise, if you had some support, whether that is from school, family, a friend's parent.....
But do think carefully before telling your dad if there is any history of violence etc. As your parent he would be the ideal person to speak to in many ways, but you know better than us what he is like and how he might react.....
Also, if you prefer not to say that you don't know who the father is, you don't have to! Just say you don't wish to speak about it .....