Is 124.2 mg/dl a normal blood sugar level for a 17 years old girl?

Blood glucose: The blood sugar or glucose level does vary depending on when it is measured relative to a meal. Normal fasting is 70-100. Normal non fasting is 70-180. You are normal if this is a random level.

Was it fasting: if yes then high. If not fasting then when was it taken in relation to a meal?

Depends: Hi. There are different normal ranges for glucose for fasting and for post-challenge (similar to after a meal, but the "challenge" glucose is 75 grams liquid glucose). 124 mg/dl is not normal for fasting, but would be normal after a meal (or 75 gram glucose challenge). Eat healthy and exercise regularly for life!