why don't you want them to know? unless you're trying to surprise them, they will see your body change overtime. you can let them know so they can help you. they can help by preparing healthier meals, and giving you healthier food for lunch for school.
if it's been more than a year, see a doctor and get a physical. make sure everything else is relatively healthy, then go over your weight loss goals. losing weight is 90% dieting and 10% exercising. you can work out all you want, but if you're still eating a lot of sugary foods, soda, cake, ice cream, etc, it won't do anything for you.
do some research. read articles online of what to eat, what not to eat. setting a calorie limit is important, but eating the right nutrition is just as important. if you have a smartphone, you can download a free calorie counter app to help keep track of what you eat.
see if your school has a nutrition class you can take that will still let you graduate on time. talk to a counselor.
talk to your nurse at school for healthy lunch options too.
see if you can join a sport. track, football, basketball, something that gets your heart rate up. that will help immensely with your diet plan.
this will take time. some people need a year or more before seeing relevant change. even if you aim to lose a pound a week, which is considered healthy, that's 52 pounds for the year. so keep that in mind. patience and consistency is key.