Being able to exercise with high blood glucose depends on how high and how much insulin you have on board. If your blood glucose is higher than 300 mg/dl, you should not exercise until you have taken some fast-acting insulin and your blood glucose level is below 250 mg/dl. The problem is this: If your glucose is high and you don't have enough insulin in your body to use the glucose, your cells will continue to signal they need glucose for fuel, and your liver will continue to put out that glucose. Without insulin to let the glucose into the cells, the glucose keeps building up in your blood, pushing your blood glucose levels higher and higher.
If you have enough insulin available, exercise may actually help lower your glucose. In fact, many athletes raise their blood glucose before exercise on purpose to create a bit of a buffer so they won't go low during their activity.