What will happen to a teenager if heart-rate increases and there is a lack of oxygen?

Depends: The body is a finely tuned machine that adapts to meet changing needs. To meet body/muscle demands during exercise, the heart rate increases. Anxiety often increases the heart rate too. Breathing will increase deeper & faster. Even at normal heart rates, lack of oxygen is not a good thing, but a teen has stronger heart cells can tolerate less oxygen for a longer period than a much older person.