How much processed sugar will make you gain weight?

SUGAR DOES NOT MAKE YOU FAT.

A SURPLUS IN CALORIES DOES.

/bangs sentence into head

Ditto

Foods that are very high in sugar are usually also very high in calories.  So it's easy to get too many calories from these foods than it is from other foods.  If you routinely eat too much food in total you will gain weight eventually.  However, sugar is not a 'bad food', is not 'evil', 'fattening' or 'unhealthy' all by itself... and can be enjoyed in moderate amounts as part of a healthy balanced diet.

You seem very confused about calories, nutrition and what constitutes a healthy balanced diet.  Check out this website specifically aimed at teens.  There are some excellent tips.....  http://www.eatwell.gov.uk/agesandstages/teens /weeklymenu/

If you aim to get a better diet rather than something ridiculous like... let's say.... '1200 cals a day', and if you tried to get more activity into your life then you'd find your bodyweight stabilised at a very healthy level

Undereating as a teen, as you are doing, is going to do damage to your metabolism and healthy, as well as making it easier to gain weight later in life - that should be your concern, not how much sugar you are eating.

There is no food that will "make" you fat.  Only eating more than you burn will make you fat. 

That said, refined white sugar is empty calories with no food value at all. 

Concentrate on getting good nutrition and the appropriate amount of calories for your age, keeping in mind that you are still growing and need more than an adult.

Ditto what everyone else said.  There's nothing wrong with having treats with sugar in them, but there is something wrong with having them instead of healthy meals.  If you don't want to eat the school lunches because they are unhealthy or unappetizing, couldn't you bring lunch with you from home?  A sandwich or something like that instead of granola bars?

3,500 cals worth. 

cheers!