Hi emarie,
I've tried skimming through your past posts to get an idea of your points use and exercise plan. It sounds for all the world like you're following the plan to a T. And since you're still in High School, it sure isn't menopause
You're eating a lot of processed foods and lowfat stuff, but the weight gain/maintenance shouldn't be water retention because you've been eating in a similar manner for the past 8 weeks. Before I officially joined WW I was following the 50 yard view (counting points but not doing the 8 healthy guidelines) and eating some crappy stuff. I still lost about a pound a week all the way down to my goal of 134 pounds (5'4").
It's a longshot, but you could see your doctor about the sudden weight gain that prompted joining WW, and then the lack of weight loss with WW. If I were you I'd bring your food journals and weigh in book for the past 2 months in with you, so she/he can see what has been going on. There might be a medical reason (hormonal, vitamin deficiency, etc.) that your body wants to gain weight.
My other idea is, since the school year is coming to an end and things will be less hectic, try switching to the core plan. I love the idea of core, the whole foods and real meats and never having to worry at the end of the day "do I have enough points for that?" Right now I'm in no mood to make my lunches, so I'm doing Flex and eating Lean Cuisines, but on weeks that I'm energetic and willing to prep lunches on the weekend, it's great!
Third thought, and then I'll shut up: you also mentioned feeling stuffed on 20 points. You're also eating a lot of low point things (lowfat bars, 2 point snack packs, weight watchers ice cream). You could eat more energy dense foods (more points with less volume) like regular granola bars, bread, turkey, etc. Admittedly you would eat less for the same number of points, but it gets around the feelling stuffed issue.
Good Luck sorting this out!
-Amy