before you do anything else, examine the food you are feeding. i'll say it now so it's out of the way most popular, highly advertised brands are poor quality, despite what the commercials tell you.
don't feed a "diet" or "weight management" food. most of them are too low in protein and fat to keep a dog healthy and just supply lots of (unnecessary) carbs.
first of all, i would definitely suggest feeding twice a day. if you feed first thing in the morning, right after getting up, you should have plenty of time to let the dog potty before everyone leaves the house. ideally you would also take him for a walk for that and not just let him out to do his business - movement stimulates the bowels. going for a brisk 10 minute walk with him before you go to school would be awesome already but if at all possible, work it up to half an hour. take it easy in the beginning, just like overweight people, overweight dogs can easily injure or overexert themselves.
feeding a good quality food and increasing exercise slowly should do the trick. go for another walk in the afternoon/evening, it will give your dog a different type of exercise than just running/playing in the back yard.
i don't know what you are currently feeding your dog or what is locally available to you, so making a recommendation is a little difficult right now. you will need to determine what would be the dog's ideal weight and feed accordingly. many people make the mistake and feed however much the bag o food tells them to feed a dog of a specific weight group - not taking into consideration that a dog who weighs 30 pounds and should ideally weigh 25 pounds should eat the amount appropriate for a 25 pound dog and no more.