No, do not listen to your coach, especially if you're a HS runner, and this is HS coach, at this age, weight shouldn't be too much of a factor as the body is still making rapid changes in development relative to what you'll have in a couple years
Losing weight is not 80% diet, there are some kids that eat nothing but junk food and do no sports but they still don't gain and weight and have a really skinny frame, why? Most of it is just genetics... If you gained those 15 pounds very easily I'd be maybe a little more worried but if this isn't the case, I don't see you having to worry about weight too much, because after a couple weeks of running the weight will take care of itself
Also it depends what type, fat or muscle, if you packed on a little more muscle this can be a slight advantage as the larger muscle mass you have the more calories you will burn per mile and it might be easier getting the weight back off... If it's mostly fat, I would try changing the diet up just a little more, maybe following a more balanced diet...
Like I said, your body at this age matures at very random moments and it may just be a results of that... I've been through the same thing, I always weigh the most in the winter... In the winter, XC is over, you've had thanksgiving, school is wrapping up, heading towards Christmas, more bad food options, you don't do as much mileage, this is totally normal, so Im not sure why your coach is over reacting, it's normal... And this winter might be a phase where your body is making that 10 pound jump... A lot of people go through it...
Just keep up the mileage, don't focus too much on workouts, just get in a good base, and you're body will naturally adjust its stride and weight accordingly to what's optimal...
If you don't mind me asking what's your age, height, weight, mileage and event specialty...
PS: I've gained 12 pounds in the past couple of months and my workouts and overall fitness has gotten better, like I said you're body will naturally adjust form and compensate for weight change