Ugh. This happened to my dad when he was 18. He was paying his own way through college and living on his own, and his parents claimed him after his first year, resulting in him not being able to secure any loans as his parents income was too high. When he asked his parents for help because they screwed his chances of doing it on his own, they refused and told him he didn't need college and to just stop being lazy and get a job. My grandparents weren't great humans.
Anyway;
A lot of this depends on what your parents are paying for.
Do you live at home? If not, do your parents pay for your dorm/apartment?
Do you pay at least 50% of your own expenses?
From what I understand, you can file your own taxes for your income tax from your job no matter what, although, if you are still considered financially dependent, if they claim the personal exemption, you can't, because that can only be claimed once. To clarify that while you're doing your taxes, you'd have to check the box stating that someone else could claim you as a dependent.
To know for sure if you're considered financially dependent and able to claim the personal exemption, more information on who pays for what would be needed. But essentially, you're under the age of 24, and a full time student, so they have those two requirements met for you to be a qualifying child. The rest is all how much you pay for vs what they pay for, and who can prove it.
If you both file, and you do claim the personal exemption, the IRS will call both parties and get to the bottom of who was right and who wasn't.
I found a thread from a year or two ago I'll PM you that may help clarify things.
Oh, and lastly, the medical stuff your dad paid for and is trying to use as justification for taking your claim sounds like he's just being a dick. It would count as expenses he paid for which could impact whether or not you're actually his dependent, but I saw a comment stating that it's "a legitimate debt."
Its not a legitimate debt the way I'm looking at it. You're on his health insurance, I'm assuming, but it sounds like he got the bill and just paid it for you (also assuming that it wasn't 100% covered by insurance and there actually was a bill- there isn't always a bill depending on your insurance or if the deductible had already been met for the year) willingly. Now, if there was ever any sort of conversation or agreement that you would pay it back, you should when and if you can, even if it wasn't a written agreement, even if only to avoid burning a bridge/not being an asshole that doesn't keep their word.
But really, it takes a special kind of asshole (which there seem to be many of) to pay for something and expect to be paid back without expressing it to the person they were paying for until after the fact. In this situation, it sounds like he paid for something without expressing that he was expecting to be repaid until he saw a way to get all of it at once from you without your permission/against you will.
Sorry for the long ramble. This situation just hits close to home since I've heard my dad talk about his life being turned upside down and also getting screwed out of a post secondary education from his parents doing the same thing.
All in all, worst case you should file your own taxes from the jobs you held that paid, because even if you're legally still a dependent, you have to ability, and possibly the requirement, to do at least that much.
I hope you get this resolved to the best of your ability. Good luck!