I am a teacher. I work a second job. The employer at the second job puts up a weekly schedule and I expect to work those hours and budget my money based on the hours. On a weekly basis, the employer calls me to tell me not to come in to work that they don't need me that night or that my hours are shortened. I've been very compliant up to this point but it is beginning to bother me. Is there a law that states if the schedule is posted with a beginning time and end time for the worker it is against the law to change it? I've been told there is.

You were told incorrectly. There is NO law in ANY state that requires the employer to provide hours just because they were scheduled.

In a very few states, a minimum number of hours ("reporting pay") must be paid if the employee shows up for a scheduled shift but is sent home before the shift ends. But that does not apply to calling you off before you show up at work, and Florida isn't one of those states anyway.