Why am I slower at cross country in the morning because I do well in after-school races but not so well in morning races?

There are several possible reasons as to why you might run better in the afternoon.
1. You may be glycogen depleted in the morning: Generally, upon waking up it's been the longest in the whole 24hr cycle since you've eaten. Even if you eat a good breakfast your body might not have time to break down all the complex carbohydrates before you begin racing. Try carb-loading the night before the race, or eating a breakfast with simple carbohydrates that are broken down quickly.
2. You might not be warming up enough: The morning is generally the coldest part of the day, and the longest time since you've done any activity. For all the metabolic, enzymatic, and circulatory pathways to be at optimum race performance your body has to warm up and get used to a little bit of exercise to snap it into performance mode. Try spending a little more time doing goal-pace warmups before you race.
3. whatever other reasons there are, I don't know or can't think of them right now... ask your coach