Anxiety and panic attacks usually happen at the most inconvenient time. You can't exactly do a 20 minute mindful meditation in the middle of an exam, but there are other things that you can do. Here are nine things that you can do if you need to calm down in a hurry—some of them you could even do in the middle of an exam:

1. Deep breathing—this one works. I know because I have used in a lot. It's quick and easy and has been proven to reduce the symptoms of anxiety.
2. Write it down—journaling is another proven way to reduce stress and anxiety. You may not have time to write a three page journal entry, but you can quickly note your thoughts and feelings to address them later.
3. Smell something calming—lavender has calming properties. You can rub a bit of essential oil on your wrist and smell it when you're feeling anxious.
4. Have a drink of water—if you're dehydrated, you can't concentrate or think properly. By having a drink you hydrate yourself, but it's also a distraction. 
5. Chew gum—this has also been shown to improve mood and reduce anxiety.
6. Relax your muscles—focus on long neck, low shoulders and loose fingers. 
7. Use positive self talk—remind yourself that you are capable and can do this.
8. Look at the evidence—remind yourself that you have overcome every obstacle in the past and can get through this one too.
9. Listen to music—it's not always possible to do this but you could listen to music before an exam or other stressful situation.