Be sure to keep an open line of communication with your parents/doctor, to make sure that you stay healthy with your lifestyle choices while losing weight!
Don't stress too too much about counting every single calorie; even the best estimates are just that - an estimate!
Personally, I'd also probably bump up your protein; I generally aim for ~1-1.5g/kg of bodyweight every day (I enjoy 30:30:40 better, myself). But you can figure out what works best for you.
Finally, bear in mind that to fill your stomach and be satisfied, veggies tend to have the best 'bang for the buck' value in terms of volume vs. calories.
I will actually respectfully disagree with novangel; jogging might not be your best choice - though she is spot on with recommending 'working out' more than once a week. This is a personal bias about running/jogging because A) I don't enjoy it
and B) It is high impact, and can be quite hard on your body. But take this with a grain of salt, k?
I like 'speed walking' for an extra 10min every day. Everyone can find an extra 10minutes a day to speed walk around the house or walk up the stairs a few times. If you have the resources at school, I'd definitely check out adding some resistance (ie. weight lifting, etc.) activity if you can do so comfortably. Or even short periods of high intensity activity.
EDIT: Desiderata has recommended against intense stuff right off the bat. I don't know enough about your personal condition to comment, but please err on the side of safety and make sure that you are progressing in a healthy way. I'll leave it to the people more knowledgeable about Cushing's (ie. Desiderata!).
Stay positive, you're definitely pointed in the right direction!