How to lose 10 kgs in 3 weeks for teenagers

Doesn't matter teen or adult... you aren't going to lose 10 kg in less than several months... without an amputation or two.

Do a crash diet expect there to be a rebound weight gain resulting in you weighing more than now.

You have to do this slowly.. or you screw up your metabolism and it makes it hard to lose it.. and far easier to gain it.

It's a genetic survival tactic.. to help humans survive in times of famine... these days it results in overweight people gaining even more weight because of improper weight loss tactics.

Not possible at all, and even if a person did not eat ( fasted) for 2 weeks, they weight loss would not have come off healthy and would cause more issues than it gained.

Also depends on how old the teen is, and what their current height and weight was to if they even needed to lose weight.

But yes perhaps in two months, or 3 is better.

But proper diet, plenty of exercise is the way to do it. No magic pills, no gimmicky

The average person needs only 2000 calories a day to maintain weight; with no loss or gain. If you are eating more than 2000 calories a day, you probably are going to continue gaining weight, unless you are working very hard, or exercising very hard almost all day long.
Most people don't realise how many calories a day they are eating. Some tell me they figured it up; to 5000 cals. A day! No wonder they are 100 lbs. overweight.
I agree that you can't lose that much weight in 3 weeks. But, Please start now to lose. If you don't know the calories in what you eat mostly, then look it up online, or Google. You can count them everyday. You would be amazed at how much you are eating!