A nice muesli bowl with nuts and fruits. It keeps you full until lunch time and gives you energy for all the coming tasks!
Mostly, I drink tea or porridge but when there's no time on my side I don't eat and just leave for school so I eat some of my lunch in the morning
A piece of fruit is always good, especially if you have to run out the door quickly. Muesli bars are also a good option, as well as protein shakes – if you have a few minutes to prepare it. I tend to prep my breakfast on the weekend by making bulk food and keeping it in the fridge where I can grab it with very little thought and/or preparation. If you set aside time in a less busy period, it saves you a lot of mucking around in the busy times, with the added bonus of it being healthy (provided you prep healthy things of course!).