I'm 11 years old and don't think I'm eating enough. Here's my main daily diet.
Breakfast: One piece of toast with 1/2 a teaspoon of vegemite or sometimes a small bowl of oats.
Lunch: We have 2 lunch breaks at my school (Morning tea and lunchtime). Morning tea: Small yogurt or on a rare occasion a chicken and salad wrap. Lunch: Nothing
Dinner: A small portion of whatever my parents make.
Is this enough for me??

No... why are you not eating lunch? If you don't like the school lunch, can you bring something from home? It also would be good to eat the bowl of oats (or something similar) everyday. One piece of toast is not enough for breakfast.

You don't need to eat a lot at each meal, in fact most people eat more than they really need to, but you should be having a steady intake of nutrients and calories throughout the day. This can often be done with breakfast, lunch, and dinner, with a small snack or two, such as a piece of fruit or the yogurt, in between.

I agree, that amount will lead to improper growth and many areas of health concern.