You can't actually "fail" a subject unless you receive N overall for the unit (which is bloody hard to do, might I add).
You have to demonstrate all the outcomes in the study design to a satisfactory standard to achieve an S - that involves meeting all the key knowledge and skills along with completion of the assessment for each outcome.
If you "fail" a general maths SAC you will be required to do redemption which will involve resitting part or whole of the assessment task (most likely another one that is similar). Your teacher should have given you this opportunity. They can also award an S if they are satisfied you can meet the outcome and have evidence to back it up.
You also don't need to pass Units 1 and 2 to go into Units 3 and 4. Schools cannot prohibit enrolment on these grounds unless it is a VCAA mandated prerequisite which as far as I recall there are only a few subjects where this applies.
tl;dr no, you'll still pass. I've seen a lot less get through with an S.