My progression during XC of each year:
7th: 24:40 -> 23:20 -> 21:50 -> 20:50
8th: 20:50 -> 19:50 -> 18:50 -> 18:30
9th: 19:50 -> 18:50 -> 18:00 -> 17:50
10th: 18:20 -> 17:20 -> 17:00 -> 16:53
Sure you can do it. As you can see, my times slowed their improvement between 8th and 9th grade, but I had a bit of a breakthrough in 10th grade. Between those two seasons of XC, my mile improved by 20 seconds, my 800 improved by 10 seconds, and my 2mile improved by 50 seconds. I didn't grow any, I didn't increase my mileage, and I didn't get a new coach. All it took was years of CONSISTENCY and the times came. I think you can get close to 17, but I would make next year's preliminary goal for XC 17:30.