Yes. I lost 17 lbs by counting calories and excersising. I did a moderate program of two three mile runs/ week and one alternative form of exercise (boogie board, hike, walk, yoga). This was during a time when I didn't work. When I started a new job and developed runners knee, I stopped altogether, but continued to eat smaller portions as I had learned to do while counting calories (I no longer counted, but had learned what was reasonable to eat). All in all I only gained two pounds back in 6 months (then got pregnant). Anyway, point is this. You can excersise until you are blue in the face, but most of your weight loss will come from eating less. Excersise is such a wonderful aid to weight loss for so many reasons: raises metabolism, helps modivate you to make better eating decisions all day, burns some extra calories, helps you tone up, uplifts your mood, etc. You will only put on weight "and then some" if you drop your calories too low (and put your body into starvation mode). I followed advice from Gillian Micheals on eating only your basal (resting) metabolism (for me this is about 1450 calories). That way any movement you do amounts to weight loss , but its enough calories to keep your metabolism from slowing. If if you excersise you loose even faster. Yes, you can use this summer to loose weight, and if during the school year you cannot exercise much, yes you can maintain you weight loss by watching your portions. I am proof of that. Here is another trick, weigh daily and anytime you gain two pounds, cut back for a few days until you are at your maintainable low. Don't be afraid to try. You absolutely won't gain it all back if you can keep your calories to about 1600-1700 (depending on your age, height bla bla bla..) Think of your body being in two modes: weight loss and maintenance. Go into weight loss mode this summer (it could look something like this: 1400 cal/day, exercise 60 min 4 times a week). Then go into weight maintinance (it could look something like this: 1650 calories/day excersise once a week). Good luck. Don't be afraid to try. Its not about dieting, its about making a permanent change. Here is another analogy: you are an airplaine who has gone off course. you were flying from NYC and are headed to L.A. However, you really want to go to Seatle. Don't make any drastic moves, change course by 3 degrees and keep with it, you'll end up in the right place.
on another note: i just bought Gillian Michaels 30 day shred. the point is its 20 min. intense work outs. So even with little to no time you can get a surprisingly great work out. She makes every second count.