As a teacher, I would encourage you to attend Saturday school. The teacher will help you with test taking skills for the Regents exam. You definitely need a math regents exam on your transcript. There are also some excellent websites to get extra help in algebra. I recommend www.regentsprep.org.
You don't have to go, but I believe you should. It would help a lot and it is really important to understand algebra because math does not get any easier after algebra but understanding the basis of algebra would help a lot.
You might want to go because algebra is in all future maths that you will take so if you are struggling now then go or else you will regret your decision in the future
Having a strong foundation in algebra is important for later level mathematics. So I highly recommend that you go ahead and take the class anyway (even though you passed) so that you can get a better understanding of the material. Its not all just about passing tests. You really need to know how to do algebra.