As a general principle, at-will employees of a private employer can be terminated for any reason or no reason at all, unlawful discrimination excepted. There are a small number of other reasons as well. You do not mention a current contract, so I assume you don't have a current contract. Similarly you don't mention a union, so I assume you're not a union member.
If you reject the contract offered to you, I see no reason why you can't be terminated, according to general principles of law. Your remedy is to find a better job that pays more. If she can find another teacher who'll accept the contract terms, then the salary in the contract is "what the market will bear."
Not legal advice as I'm not licensed in Florida. If you need legal advice, please consult a lawyer who holds Florida licensure.