If you are in poor health, write a sick note and take good care of the case. Some heart diseases and the like can't be exercised vigorously. Cardiovascular diseases, obesity, gallstones and other diseases can not run and exercise. In other cases, if you don't want to run exercises, you can ask the class teacher for leave.

If you don't want to go just because you are lazy, it depends on how you handle it, or suggest exercise. Health is the capital of revolution.