Football is generally a much more demanding HS sport than XC, so you're better off prioritizing that and fitting in XC training and racing around that. It's much easier to miss a XC practice or race than a football practice or game. You'll need to be honest with yourself and your coaches about how much time (and energy) you can dedicate to both without over-extending yourself. Talk to your XC coach about how can be most efficient with your training. Coaching is a mix of art and science. A good coach will know what your strengths and weaknesses are as a runner and then develop a training plan to address your weaknesses and preserver your strength. You should know this for yourself as well when you discuss this with your coach. An extreme example would be to just run the XC races with no training beyond football practice and your base fitness. You want to avoid running a lot of miles (while playing football) because you could easily over-train and get fatigued and not race well. Try to identify some short, quality work that you can fit in when you have the time.