Before I respond, I need to that that I am a lawyer with some understanding of the legal issues, but I have not had the opportunity to examine all of the facts, nor the law in your state, and I have no experience advising teachers.
The situation you lay out is disgraceful, and you need to be prepared to fight for your rights.
First of all, did they give you anything in writing to say that you've been hired and the basis on which you would be employed? If so, did it provide any "out" for the district - did it say this was subject to school board approval or anything like that? If they gave you a writing setting forth the terms and it did not expressly say that your hiring was truly conditional (the mere fact that they needed to prepare a formal contract does not make it conditional), then I think you have an enforceable contract.
Of course, what you received may say that your employment is "at will," which generally means they can terminate it at any time, for any reason or no reason, but on the other hand they could be liable for "reliance" damages for offering you employment that they had no authority to offer you - particularly where they expressly told you that your offer would NOT be impacted by the hiring freeze and you interviewed with everyone in the district - it's not like they can claim some junior person offered it unaware that there was a hiring freeze.
Some of this may seem like gibberish to you. So, in short:
The school district screwed up and you may have a basis for legally challenging what they are trying to do.
The first thing I would do is go speak to HR and make sure they are fully aware that their failure to honor their offer means you must begin repaying loans, can't get tuition reimbursement, and don't have healthcare, and you're very upset about it, particularly since they expressly told you that the hiring freeze was not an issue. If you just rollover, they will let you roll. If you push back, they may have difficult conversations with higher ups to make this work out for you.
Does your district have a teachers' union? Even if you are not technically in one yet, they may offer advice.
You should consider hiring a lawyer to give you advice specific to your situation.
You seem to have a great head on your shoulders and to be a remarkable educator who will be in demand elsewhere. I do not think you should begin a career on this basis - where they tell you one thing and then completely pull a bait and switch on you in the first week - this is disgraceful behavior and if you put up with it, they will know you'll put up with anything. They hired you full time, for a decent salary and full benefits. It is unimaginable that they could flippantly tell you that you have to work for a lower salary and no benefits for 8 months first, and if they insist on this, they don't deserve you, and you deserve better than them.