Is it permissible for a female student to greet her male teachers in the street, and is bowing her head upon meeting them without offering a greeting considered unethical behavior?

The basic principle is that women should teach girls. If a female student's greeting her male teachers causes temptation, then it is not permissible. It is permissible when there is no fear of temptation, such as when the man is old. If the student avoids greeting them, this is part of the praiseworthy modesty.