Title VII of the Civil Rights Act explicitly prohibits discrimination because of sex in matters of employment. Florida is free to prevent teachers from teaching things, but they cannot fire people for their own sexual identity, per federal law.
Meanwhile, Title II of CRA covers interstate commerce and prohibits discrimination because of race, color, religion, or national origin - but not sex. Under Federal Law, if your business has a lot of out of state customers (primarily hospitality) or includes supply chains that cross state lines, you can’t discriminate on race, etc but you can discriminate on sex.
The 14th Amendment states that the law must apply to everyone equally. However, this only applies to governments (and their contractors) - a black person cannot be refused to be heard in court and a gay person cannot be refused a marriage.
The way US law is supposed to work is that states can set their own laws where Federal Law doesn’t cover it. However, they must do this within the bounds of Federal Law. This is why we have 1st Amendment challenges against state laws that fill in the gaps of federal law - a business can discriminate based on sex, or any other reason (so long as they don’t fall under Title II), even if state law says otherwise.
US law is so shit. It’s unnecessarily hard to read, distributed across multiple yet interwoven jurisdictions, and full of holes. But hey, at least it isn’t financial regulations - reading those will cause a sane person to lose the will to live.
TL;DR This should be a slam dunk for the teacher, per Federal Law: Title VII of the Civil Rights Act, which overrules anything the states write. However, who knows how the current Supreme Court might try to spin it - if they even opt to hear it (they absolutely should).