When Ruth (not her real name) asked to sign up for an ultra-Orthodox high school in her home town in the North, she was refused.

The official reason: Claims by the educational institution's management of so-called immodest behavior by her mother - which Ruth rejected out of hand.

In a letter written by the principal of the school a few days after the school year opened, while Ruth was still sitting at home, the principal revealed a list of conditions for accepting Ruth - all of which apply only to her mother.

In the end, after almost three weeks at home, Ruth was accepted "on probation" to the ultra-Orthodox girls high school, the conditions being that her mother keep to the requirements listed in the letter.

According to Rabbi Yoav Lalum, claims of immodest behavior are usually a front for justifying discrimination. Lalum is the chairman of a non-profit organization battling ultra-Orthodox schools over sectorial discrimination against Jews of Sephardic heritage.

He added that his organization is now handling about 100 cases where ultra-Orthodox educational institutions have refused to accept such students.

In a letter obtained by Haaretz, the principal writes that Ruth would be accepted "but only on the condition that her mother accepts upon herself all the modesty restrictions including all their details, everywhere and always. It is important to note that if all the rules, or any of them, are not followed, then her daughter will not be allowed in the school, and the high school's management reserves the right to suspend the student if necessary."

The letter then details five general rules for Ruth's mother: "Completely covering her hair; shirts - not tight or with printed phrases, closed at the neck and covering the elbows; skirts - not tight, without slits, and jeans material is not allowed; socks are required and must cover all the exposed leg; and all fashion accessories must be modest and not stand out too much."

Other requirements demanded by the ultra-Orthodox high school include: "The atmosphere at home and what is said about the high school and those who work there must be positive." The school further demanded that nothing should be said that was not in the spirit of the school.

The conclusion of the letter included a statement from the principal that "it is important to note that the student is still not registered and her place [at the school] will only be guaranteed after all the conditions are fulfilled, and the high school's management is convinced of the change.

"If the expected progress does not happen, the student will not have a place in our school. If the matters are corrected for a period of time and afterward there is a relapse [from meeting the conditions], the student will removed from the school."

According to the principal, the list of conditions was accepted by the extended family.

Others in the ultra-Orthodox educational system said that often parents are willing to compromise and agree to such requirements so that their children can study in a well-respected ultra-Orthodox institution.

According to Lalum, it is easier for such institutions to disguise their racist motives for not accepting such students by claiming that the mothers are not modest enough. The situation is much worse in secondary schools for girls, since in elementary schools the local authorities can intervene. He added that his organization plans to petition the High Court of Justice on the matter soon.