I think that you should take your Mexican textbook and throw it out the window. Utilize authentic native language materials for your classroom. This way, you will never have to encounter interference problems again.
I believe that your textbook was written by a person from Mexico that had a greater proficiency in Spanish rather than English, the target language for your course. "To do exercises" is highly likely to be just a bad translation from Spanish since hacer ejercicios
is more common than using the verb ejercer
In English, we would tend to opt for the verb "exercise" itself, unless we wanted to stress plurality or distinguish the exercises from one another by using a countable noun, which in this case would require using "do" or "does" depending on the subject, of course: he does a variety of exercises every morning, for example.
If you would like to learn more about how to make your own textbook for your language classroom or you would like to learn more about the Spanish language, feel free to visit me inside my on-line school.
Eric Paul Monroe