Have a goal of 2lbs/wk loss. That is about all that can be lost healthily w/o sacrificing muscle tissue/nutrients.
So at 308, that would be 64 weeks, or a little over a year to get to 180. Unfortunately results dont come overnite. You could set a goal to be in shape before you go to college, and then start taking kempo classes at university.
what alot of the so called weight loss gurus dont tell you is that weight loss is a simple math equation, you must burn more calories then you consume relative to your BMR (basic metabolic rate).
Say your body requires 2500 calories/day at your currnet BMR. You have to create a calorie defecit, so if you walk for half hour (burn four hundred calories) and consume 500 calories less (2000/day). You would have a 900 calorie deficit that day. approximately Every 3200 calories of deficit is a pound of weight lost.
like vegetasama said, i would incorporate weight training in to a routine. Start out with 3x week. Focus on the legs (squats) and bench press (chest). these are compound movements which will build the most muscle mass. The legs and chest can get much bigger by volume than say your calves or bi's/tris
the goal is not necessairily to bulk up, but to increase muscle mass as readily as possible. Each added pound of muscle will raise your BMR by about 100 calories. So in the previous walking & eating less example, Instead of a 900 calorie deficit you would have a 1000 calorie deficit because you are consuming 2000 calories, and burning 400 and you need 2600 to maintain that BMR.
Dont panic if your overall weight goes up intially, your body has to store and build up the new muscle tissue before it becomes an efficient fat burner.
Also dont get hung up on the numbers, beacuse they are arbitrary and used for example, if you understand the principles of weight loss you dont need the numbers.
So I would start with eating less (just a couple hundred calories less than what you currently eat) and lifting 3x / week. For now, just chest and legs (you can incorporate other muscle groups later). Also start walking for (work towards 30-45 minutes) in the mornings before school (you burn more fat in the am, because your other energy stores have been depleted). The other benefit of lifting with your legs is to build them up so you can support your frame. You dont want to jump right into running, because your joints and muscles arent conditioned to support your weight yet.
Hope that helped, let me know if you have any more questions,