It's an unfortunate opinion which says something about their attitude toward nursing. Many working students are concerned whether or not they should let their boss and co-workers in on their secret. This is a highly individual decision which depends greatly on your work arrangements, how much school will affect your work, and how well you get along with your boss. Some bosses are supportive, but bear in mind that a boss's job, as well as yours, is to act in the best interests of the firm. It's easier if your going to school has a minimal impact on your job performance.
Employers also vary greatly in the flexibility regarding time off.
Be creative in using your vacation time. You might only need half a vacation day to attend an extended clinical or you might skip lunch to leave an hour early for last minute preparations for a test. You may have to take a shorter vacation to save your vacation time for school activities that might conflict with your work schedule. Use your lunch hour to study or even get in a little lab practice if your job is close enough to school. The student who is already working in health care has a distinct advantage.