The ad appeared in the Herald, the Daily Mail and the Scottish Mail, with the heading: "Hottest back to school fares".
Underneath the heading was a picture of a teenage girl standing in a classroom, wearing a version of a school uniform consisting of a short tartan skirt, a cropped short sleeved shirt and tie, and long white socks.
The copy stated that one way fares to Derry, Belfast, Budapest, Grenoble and Stockholm (Skavsta) were £10 including taxes and charges.
The Advertising Standards Authority received 13 complaints from members of the public who found it offensive.
Ryanair claimed that the ad did not suggest sexual connotations. It said that it believed it was obvious that the image was of a woman fully clothed and that the short skirt and bare midriff were representative of the type of clothing that was fashionable among young women in the UK.
The ASA considered that the model's appearance and pose, with the heading "Hottest" appeared to link teenage girls with sexually provocative behaviour and was irresponsible and likely to cause serious or widespread offence.
Ryanair was told to withdraw the ad.
The airline also came under fire from the watchdog this week following a single complaint about a separate press ad, which stated "The lowest fares from Britain. £10 all in!".
A reader challenged the availability of flights at the advertised fare and the ASA challenged whether fares to all Ryanair's destinations were available at £10.
Ryanair supplied examples of bookings on flights to Barcelona and Bratislava, the two destinations cited by the complainant during the time of the offer. The tax-inclusive price in both cases was £10.
The ASA noted that the ad qualified the offer with "Subject to availability, terms and conditions. Flights direct from selected Ryanair UK airports".
However, it also noted that the Committee of Advertising Practice help note on travel marketing, stated that if not all seats were available at the quoted fare, ads should state prominently that prices were "from".
The advertising watchdog told Ryanair to amend its ad to state that the price quoted was a "from" price.
In a busy week for Ryanair, it has also been forced to apologise over a newspaper ad in Le Parisien, which featured Nicolas Sarkozy and Carla Bruni.
The caption next to a smiling picture of the French President and his model/singer girlfriend, read: "With Ryanair, my whole family can come to my wedding".
David Martinon, Sarkozy's spokesman, said: "We are looking at all possible legal avenues because this is unacceptable."