1) Can what I say be used against me in court if this was recorded without my knowledge?
Yes, you spoke publicly and incriminated yourself.
2) Did I admit too much or will this conversation be deemed invalid due to too many non-solid answers?
Non-Solid answers make you look more like a lair. You admitted to the crime and the details don't matter at all.
3) Will they even bother fully going through with the case if what I assume is true and they don’t have surveillance, only our words?
Yes.
4) Is there a possibility they recorded this without telling me? And if so, is it legal to use in court against me if I wasn’t told about the recording? (I was not under arrest during the interrogation, they were just questioning me and no, they didn’t read me any Miranda rights or any of that since that’s when you are being arrested.)
You weren't under arrest but what you said can be used in court. It doesn't matter if you knew that you were being recorded or not.
5) If they come back for more questioning, although I’m a minor, do I have a right to not speak to them or not go to their office? Or can they arrest me?
You didn't have to speak to anyone about it. I'm afraid it's too late though you already admitted guilt.
6) Why did they come to school to speak to me when this didn’t concern school? They already knew who we were, could it of been the fourth persons father trying to get the detectives after us?
Where I live every school has a police officer so it's not uncommon for the school to talk to you/the authorities.
7) Can or will they get a search warrant for my Craigslist account?
They already found enough information and you admitted to owning it ec.
P.S. Stop doing/hanging out with people who do petty theft. Fucking idiot.