The short answer to your questions is NO.
The long one is this... I must assume from your question that you do not know if your teenager is a diabetic or not. To determine this there are 2 simple tests a Dr. can do. First is an A1c test. This simple blood test measures blood sugar levels over an extended period of time - approximately 3 months - even though it is only taken once. The second test is a GTT (Glucose Tolerance Test) this test measures how quickly a persons body responds to a given amount of sugar ingested. This tells the Dr. if your body is producing and using its Insulin normally or not. Also it should be noted that you can buy an A1c test at the pharmacy instead if you like.
The other part of your question centers on normal readings. For a person that does not have diabetes readings should be in the normal range all of the time. Normal range varies from person to person but generally falls within the range of about 80-160 depending on the times and conditions leading up to the test. For example large meals right before the test will make it higher and exercise will make it lower. In general tests to determine whether or not you have diabetes and need further testing should be done after a 12 hour fast - ie first thing in the morning. If these tests are normal that indicates a very great possibilty that all is normal.
This begs the question as to why you are concerned and if you took a reading what it was ...
Your question really is a bit unusual... did this teenager have a blood glucose meter reading after a workout? And if so, why?
Some more info or background would really help us to be able to answer your question.
In general, though, any physical activity will help lower and/or control blood glucose in diabetics, and physical activity might show low blood sugar readings in immediate post-activity testing for any person who is not diabetic...
In terms of "diagnosing" diabetes, that really is best done with fasting blood glucose levels. (Generally in the morning after not consuming anything except small amounts of water for at least 8 hours). Ideally a fasting blood glucose would be under 100-110 (for most USA labs and the units/measurements they use, though individual labs sometimes use different standards), though the "official" diagnosis of diabetes comes about if the result is over 126. (and again, that is for FASTING for at least 8 hours).
On the other hand, a "high" blood sugar level after a workout *could* mean a number of things, especially in a teenager, but you'd have to know the level before the workout, and whether or not anything else was going on... general diet, any food items consumed before the workout, other health issues, any supplements...
If you can provide more info, I'm sure forum members here would be willing to provide more insights...