The popular carrier has been criticized for a campaign aimed at marketing low price fares to coincide with the start of the school year.
To drive home the point, a smiling "schoolgirl" in a mini-skirt and short blouse is depicted beside a blackboard announcing the "hottest back to school prices".
According to ERK, the woman in the school uniform "is used to catch the eye in a sexual manner that is offensive to women in general".
Ryanair declined to comment on the campaign when asked for an explanation by the Trade Ethical Council against Sexism in Advertising.
ERK is a self-regulating, private body that has existed since 1988. The organization bases its decisions on the ICC International Code of Advertising Practise, but has also added three supplements to the ICC's Article 4 dealing with sexism.