Tell your parents that your house is haunted, and that by being haunted every night, you can't sleep, you can't study, you can't do anything right. Then to make your point, spin your head around 360 degrees.
Research an Einstein equation and write that first on your note. Then say that you are failing math because you are capable of doing this kind of math, but your teachers just aren't challenging you.
Explain that you had been abducted by aliens, and were replaced by a rather stupid clone. Now that you're back and have gotten rid of the clone, you should do a lot better in math.
Or say you didn't study enough in math this semester, and that it has taught you a valuable lesson in that you get out of something, that which you put in. You will try harder next time, and to prove it, you will spend less time on the computer writing threads.
It'll be much worse if you don't tell them. Ive taken my quarterly progress notes, and shredded them when i found out i was failing. At the end of the quarter, when my parents got my grades, and questioned why they didn't know, they called the school. When they found out that i hid it from them... lets just say it didn't go well. I ended up not being in marching band, and left with my time at home spent staring at the 4 walls of my room with nothing to do is stare, think, read, and study for at least a good month.
Always balance out the negative with the positive.
1) Dump it on them at a happy time, and when there is other people around. (IE; people who your parent's won't flip out around)
2) Make sure you have something good to go with it. If you failed math class, make sure to emphasize how well you did in your other classes, or something equally as good.
3) Don't do something immature, like blaming it on the teacher. Make sure you let them know you take full responsibility for your mistake, and tell them you'll do much, much better. Oh, and make sure they know you learned your lesson.