Can Strattera cause aggression

No, but ADHD can cause impulsivity, which can be expressed as combativeness.

You have to ask yourself whether you are seeing the age of the child, the ADHD in the child, or a combination.

And pick your battles wisely. You don't really "win" with teenagers.

If you still feel this is an issue, discuss it with your doctor--you are your child's best advocate.