The personal network of relationships is structured in circles of
friendships, that go from the most intense relationships to the least intense
ones. While this is a well established result, little is known about the
stability of those circles and their evolution in time. To shed light on this
issue, we study the temporal evolution of friendships among teenagers during
two consecutive academic years by means of a survey administered on five
occasions. We show that the first two circles, best friends and friends, can be
clearly observed in the survey but also that being in one or the other leads to
more or less stable relationships. We find that being in the same class is one
of the key drivers of friendship evolution. We also observe an almost constant
degree of reciprocity in the relationships, around 60%, a percentage influenced
both by being in the same class and by gender homophily. Not only do our
results confirm the mounting evidence supporting the circle structure of human
social networks, but they also show that these structures persist in time
despite the turnover of individual relationships -- a fact that may prove
particularly useful for understanding the social environment in middle schools.Comment: 10 pages, 7 figures, requires wlscirep.cls, jabbrv.sty,
jabbrv-ltwa-all.ldf, and jabbrv-ltwa-en.ldf (included