How can a teenager (middle school) lose 20 pounds quickly?

Uhhh, why would you need that to happen? Don't do anything...

I usually don't respond to these questions but Ill let this time be an occasion.

Despite how convenient it may sound, losing weight "quickly" would probably not feel as rewarding for long. More like a short lived impulse that after it happens, turns into a down feeling. If you take the process duration and plan it you will feel happier once you accomplish it. Knowing your planned determination was a result of doing properly (long route) it also builds a sense of control once the results progress.

A lot of people have told me they felt that "now what" feeling after quickly rushing off their weight goals like you're talking about. I think if you want to, you can lose 20 pounds. But can you find a way to urgently cheat through it..? Probably not. Im sure its possible though. Maybe you could.

Just don't accept any easy routes as truth. Most of stuff you'll hear online will be lies and unsafe and the other 20% scams. Id say 80% of the methods you will hear online are false.

I have heard a lot of them myself and I have not once tried to lose weight myself! That, Ironically, is why Im warning you. Because with all the bs I have seen then I could only imagine what someone searching for intentionally will find.

Im sure everyone does not agree with me, but this is my advice. Besides of course the "ask your doctor" thought response.

Salads and exercise

Talk to a doctor!! Please don't do anything dangerous!!!

You didn't gain it quickly, so don't expect to lose it quickly.  The proper way is to eat a well balanced, well portioned diet. Stay hydrated and get at least 8 hours of sleep.  Also get 30 minutes of exercise everyday. Just walk. It is free and easy to do. If you don't have 30 minutes at one time, do two 15 minute walks or three 10 minute walks.

Create a Calorie deficit.

Figure out what you are consuming daily and reduce it by 200 to 300 calories if you are maintaining weight, and not gaining weight.

Add in at least 10,000 steps a day and you will lose weight. Since you didn't gain it overnight, do not expect to lose it overnight.   

I believe you should focus more on eating healthy and getting proper nutrition than simply losing weight. You will lose weight along the way, which is good, but to strive to eat healthy for the sake of it is more of a lifestyle than a goal. If you tried to just lose weight, then it is likely you will gain it back once you've lost if your goal is a particular destination rather than an overall health habit change. Also, just because you're eating enough to lose doesn't necessarily equate to eating good.

Additionally, it is important to know that even though items may have the same about of calories in them, doesn't mean that they're equally as fattening contrary to popular belief. You actually burn foods like almonds faster than crackers even if you intake the same amount of calories of either.

Also, stay clear of "diet" foods. Special "K" chips, fat free cookies, etc. They're actually worse for you, and in order to make them "fat free" or "sugar free" they either have to load in tons of sugar or artificial sweetener, which is the real devil here. Eat some real food and not factory-crap advertised as good for diets.

(Added) Sugar is the biggest perpetrator in obesity. It does not carry nutrients or anything humans need in it, but it is very fattening, addicting, and causes cravings and hunger. In fact, sugar is more addicting than heroin, which can perhaps paint a picture on the extent sugar is what causes cravings and misleading hunger signals, making it detrimental to our health. So try to limit sugar, yet don't get sweets and junk food that is labeled "sugar free." They contain artificial sweeteners, which is worse for you even though they don't taste nearly as good.

In order to lose wait follow these steps:
Consult with a medical professional
Determine if you actually need to lose weight, and if so, how much
Make sure you’re doing it for the right reasons
Be realistic about losing weight fast
Make your health your priority
Limit restaurant meals
Get plenty of exercises
Drink a lot of water
Stop drinking your calories
Avoid fad diets