Here is where I'm going to sound odd:
Breakfast doesn't necessarily matter.
(Cue gasping of the audience, and the sound of cereal bowls clattering on the floor.)
By the by, a majority of the initial studies that looked at benefits of breakfast (for children, specifically) were funded by cereal companies. Take that as you will.
Back to my first statement!
Are you hungry in the morning before school? I never was in high school--and in fact, only felt hunger shortly before the designated lunch hour. (Which was really early? Something like 10am, I would get up around 6:30 for school.) To contrast, I had a friend who would feel sick if they missed breakfast because they woke up late--and often had a snack in the hallway between our first classes.
In their case, they were unable to focus if they didn't eat before they came to school. I had no problem.
Now, I have to eat within an hour after waking up...but even then, I don't have to eat a lot. I typically have two eggs and a piece of toast. Sometimes, I have fresh fruit and a slice of turkey. It isn't until mid-day that I get really hungry, that much hasn't changed!
You still need to eat to function. The thing is, your body will tell you when you're hungry. You'll notice you get sluggish if you should have eaten, for example.
That said, if you don't have a lunch hour until much later after you wake up, especially if you can't eat something in the halls, then I do advise you at least have the granola bar.
(As far as nutrition goes--be careful. Many granola bars are low in fiber and protein and high in sugars!)
If you have the time, having some fruit, eggs, toast, cottage cheese, oatmeal, etcetc in the A.M. would be better. Food high in protein and fiber do keep you full longer than foods without!
And of course--stay hydrated! Have water before you go to school, while you're there, and when you get back. Dehydration impedes your body's ability to function, and no amount of breakfast foods can help that!
(I am not a certified nutritionist--yet?--but I have had to get into all this because I took up weightlifting/bodybuilding...which is as much about nutrition as fitness.)