I didn't have it quite that bad, but vaguely similar. My 6th grade math teacher realized I didn't need to be there and assigned me self-paced algebra instead. I was lazy, but eventually worked through quite a bit of the book. Then 7th grade came, and I was back in pre-algebra, before 8th grade had algebra again. I dealt with the boredom by reading novels through all of 7th and half of 8th grade (before it got ahead of where I had been) math. The teacher for 7-8 had mixed feelings, sometimes just letting me space out, other times pestering me to pay attention. She at least liked me, and supported me in the after-school math program, where I was an enthusiastic participant.
Random mathematical inquiry that you might enjoy: I once spent a road trip mucking around with a system of turning multiplication problems into subtraction problems, using the average and difference of two numbers and their squares. Perhaps better explained by example: I noticed that a pattern held where if you started with a number and squared it (for instance, 8 x 8 = 64) but then shifted the numbers up and down by 1 (9 x 7) the product was 1 less than the perfect square. If shifted by 2 (10 x 6) the product was 4 less, if shifted by 3 the product was 9 less (11 x 5 = 55), and so on, and this pattern held no matter what number you started with. Maybe a pointless trick, but a neat pattern, and I figured maybe someday I could win a bar bet by knowing that 254 x 258 is exactly 4 less than the square of 256 x 256, or that 195 x 205 is 25 less than 40,000.
It doesn't work so well if the numbers aren't an even number of steps apart, though, so most of the time was spent inventing placeholder techniques to compensate. For example, with 9 x 6 do I drop down it to an 8x6 problem and then make a note to add back in an 8, or do I bump it up to a 9x7 problem and then make a note to subtract the 9?