Many schools require that you sign a contract, typically for one year. This obligates you to pay for this entire time. There are some things you will want to consider before signing:
Contracts give the student some extra commitment. This is important because more people will quit jiu-jitsu in the first year than will make it to the end of first year. And if you can just make it to blue belt, you will have a set of skills that will serve you well for the rest of your life.
Contracts are also very helpful for the school. Most schools are businesses, that at least need to pay the bills, and the principle instructors. Contracts allow for a more steady cash flow. Also, new students require a lot of effort in the beginning, and contracts allow the school to be sure to get some sort of payback for the extra effort required for new students.
If you sign a contract, you are legally and morally bound to pay the entire amount. If you car breaks down, you lose your job, need to move, get hurt, or just lose interest in jiu-jitsu, you are still bound to pay the entire contract. Some contracts and/or schools will suspend, or even more rarely, cancel your contract for some reasons, but some will not, and you are usually just stuck paying regardless of why you need or want to stop training.
Most schools (not all) sell their contracts to finance companies for a lump sum. So, the school then has no say about suspending or canceling your contract if you lose your job, etc. These finance companies are ruthless. If you lose you job, or cannot pay for whatever reason, they don't care, they just want their money. And they will unleash the storm of hell in collections with brutal constant phone calls, letters, and lawsuits until they are paid. They will also report you to the credit agencies, which will strongly damage your credit.
Negotiation and Final Thoughts
Just remember, a contract is always negotiable, and you can ask that various clauses be added to cover injuries, job loss, moves, etc. Most schools will already have or will agree to injury and moving clauses, but not to job loss, car issues, or loss of interest. So, if you do sign a contract, just keep in mind that you are making a big commitment that you may or may not regret later.