How can I lose weight if my job is to sit down and teach for several hours?

In vs Out. Losing weight comes down to a simple (but frustrating) concept: calories taken in vs calories burned off. So yes, having a job that requires us to be more sedentary does make it a little more difficult to lose weight. But not impossible! start with sleep. Getting to bed earlier will help you get up earlier and have a half hour for a morning workout. Keep veggies or almonds nearby. You can do it!
Exercise, eat less. To lose weight, decrease 500cal/day to lose one pound/week. Exercise 150min/week (30min/day-just walk). You can increase non exercise activity by getting up every hour to walk, walk up the steps, park far away, pace while on the phone-this activity all adds up to help burn calories. Drink more water and eat nutrient rich foods but calorie low like fruits/veggies, lean meats.
Mix it up. You don't have to sit to teach, you can stand and walk around while teaching. There are also many exercises you can do while sitting. The whole idea is to be innovative, always thinking about ways to expend energy in your current situation. You can also add a walk at lunch... Your job add a little bump in the road but it's not impossible.