Are you constipated as well, or is your biggest problem the cramping? Has your doctor tried you on an antispasmodic, like hyoscyamine? Even if you've tried one and it didn't work, there are a lot of different ones out there and they are very hit or miss--they either work wonderfully or not at all; it can take some trail and error to figure out which one will work for you.
If you're having constipation problems, have you tried a daily regimen of prune juice? Also, there are aloe vera capsules that you can get in the vitamin section of any pharmacy; they are a natural laxative. If you can get things flowing regularly, that will often help the cramping (although not usually cure it completly).
Also, there are foods to avoid when you are cramping. One is apples. Some people can tolerate cooked apples okay, but raw is always a no-no, as is, especially, apple juice. For some reason, this cramps up people with IBS worse than anything else, it seems. Caffeine and carbonation are another two big cramp contributors, so no more cokes or coffee (not even decaf--it still has caffeine in it). Have you tried a dairy-free and/or gluten free diet? Sometimes allergies or just intolerances to those things can make you cramp and make you have constipation or diarrhea. Note that dairy-free is not the same as lactose-free and that the blood test they do for Celiac's disease 1) isn't terribly accurate anyways, and 2) doesn't mean you don't have some sort of intolerance that doesn't show up on the traditional test. The best way to figure out if you have a diary or gluten intolerance is to just eliminate it from your diet for a couple of weeks and see if you improve any. Don't eliminate both dairy and gluten at the same time, though, becuase if you get better, you won't know which is it that actually bothers you. Unfortunately both of these items are VERY common in prepackaged foods. "Caesin" is a milk protein that you will see listed on some products; you can't have that while trying dairy-free. You will probably find you can't eat anything in your school's cafeteria while you are trying these elimination diets. You will need your mom to fix you some meals from scratch so you can avoid these things. Like I said, give it two weeks to see if you have any improvement.
We had a guy on here before who was in high school and he had D problems, especially in stressful situations. He ended up going to school through an online class program and meeting as part of a study group with the other home-bound kids for a couple of hours a couple of times a week. His guts, although not perfect, were much better when he didn't have to worry about going to school and his grades were coming back up nicely. This might be something you do want to consider doing until you can find something that will reduce the cramps enough that they're at least tolerable. If kids at school ask what's wrong with you, tell them you don't know, but they suspect that you might be allergic to some sort of food, but they haven't found out what yet. That's close enough to the truth and something that most kids can understand and not ridicule you for.
One other thing to consider is some of the new antidepressants they have out on the market. They're starting to use them to treat people with IBS. I don't think I've heard of them being used on people with C, but they think that some people's cramping and D may be caused by just overactive, overly sensitive intestines and the antidepressants mellow them out so they function normally. If your cramping isn't caused by being perpetually backed-up, then the antidepressants may help calm your guts down.
Also, try probiotics as Lil mentioned. Those have helped a lot of people, at least in some small way. If you don't suspect a milk allergy, then you can try a couple of containers of Activia yogurt a day and work your way up to Digestive Advantage or Culturelle. You can find those in the bowel prooducts section of most chain drug stores. Like fiber, they can cause some gas initially, so don't take more than one a day at first. I usually take them right before bed so I can sleep through most of the gas.