You are in high school, so you are still young. There is nothing wrong with diversity in training. Here are some things to consider:
1. If not running cross country, what would you be doing? If you wouldn't be playing another sport, or not in an otherwise structured training program, then running cross country is certainly a better alternative. (i'd personally pick soccer as a better fall alternative)
2. Will you be running a "true 5k training program" or can/will it be modified? Can you have 1-2 days per week dedicated to acceleration and maximum velocity training? Then perhaps 1 day of "cross country workout" and a couple days of ez runs.
The fall is a time for GENERAL training for 400 meters. General training is pure speed, and the kind of endurance that might be equivalent to 2-mile pace.
High school rules don't always allow for "track and field practice with a track and field coach" in the fall. So modified cross country is a solid option. Full out traditional 5k cross country training is better than nothing.