Generally, principals work full time. However, they also sometimes work in the evening to meet parents and other members of the community and to attend school functions, such as concerts and athletic events.
Many principals work year-round and do not have summers off, even if students are not in school. During the summer, principals prepare for the upcoming school year.
Source: Bureau of Labor Statistics, U.S. Department of Labor, Occupational Outlook Handbook, 2012-13 Edition