To distinguish between everyday worries and the clinical presentation of Generalized Anxiety Disorder is a question of severity. Some anxiety/worry is healthy in that it's part survival instinct...this is why we look both ways before crossing the street or why as a species we don't (typically) engage in exceptionally risky behavior. Anxiety is like an alarm system for our bodies, it notifies us when there's a threat in the environment that we need to pay attention to. Typical anxiety looks like this: the alarm is triggered, we evaluate the situation and recognize that its either a true emergency or false alarm, then act accordingly. Anxiety lets us know if we need to employ the fight-flight/freeze system.
The clinical presentation of anxiety differs from every-day anxiety in that the alarm is triggered for non-emergencies as if they were true emergencies, and the individual isn't able to identify the differences. So when the alarm gets stuck in the "on" position, we have Generalized Anxiety Disorder, where even the most benign situations are experienced as huge threats. Another characteristic that distinguishes non-clinical anxiety from the clinical presentation is how much time is spent focusing on the anxious thoughts. In the clinical presentation, the individual can't stop thinking about the worries and/or finds it very difficult to shift their attention to something else. The worries can take over their entire day, making it difficult to pay attention in school, can cause increased irritability, and cause some to shut-down.
One of the best ways to "reset" that alarm is to do a reality check. Identify what's triggering you and interrupting the worrisome thoughts by inserting more helpful thoughts. For example, anxious thought: I'm going to fail my math test, I'm terrible at math, I'll never graduate high school. Helpful thought: I've done ok on my other math tests, I did my best studying, I can get extra help after school.
Often, anxiety is triggered by the sense that the individual isn't in control--so stopping to identify what aspects of the situation you can control (vs. focusing on what you can't) can help alleviate some anxiety as well.