  To remove a cognitive dissonance in interpersonal relations, people tend to
divide our acquaintances into friendly and hostile parts, both groups
internally friendly and mutually hostile. This process is modeled as an
evolution towards the Heider balance. A set of differential equations have been
proposed and validated (Kulakowski {\it et al}, IJMPC 16 (2005) 707) to model
the Heider dynamics of this social and psychological process. Here we
generalize the model by including the initial asymmetry of the interprersonal
relations and the direct reciprocity effect which removes this asymmetry. Our
model is applied to the data on enmity and friendship in 37 school classes and
4 groups of teachers in M\'exico. For each class, a stable balanced partition
is obtained into two groups. The gender structure of the groups reveals
stronger gender segregation in younger classes, i.e. of age below 12 years, a
fact consistent with other general empirical results.
