Well, I think the general rule is that on average people lose 1/2-2 pounds/week. Typically it translates into 1% of our body weight. So if you weigh 200 pounds, you might lose 2 pounds/week. But at 150, only 1.5. These are gross overgeneralizations, of course. Often people lose more weight at the beginning, though that is more water and less fat lose. But larger people tend to lose more pounds/week than smaller people.
My concern is that if you set a goal of 25 pounds and don't make it -- say you do lose 10 -- that you will feel bad about yourself and your success. Losing 10 pounds is great -- but your goal is to KEEP it off, right? So you need to begin to build good habits that will last you into the school year!! Then maybe you lose MORE weight during the year and keep it off forever!
So, I would put less worry on the number on the scale. Scales are
and don't always cooperate. Instead, focus on building diet and exercise habits that will give you a good shot at maintaining the lose (and losing more) during the year.