The Bible teaches us that those who divorce and remarry are committing adultery against their former spouses. The Catholic Church does not allow divorced people to marry in the Church or accept sacraments. Should the school hold the same standards to these people as they do towards homosexuals?
When a lesbian couples presents themself as a couple, there is no other conclusion that the children could reach other than that they are a lesbian couple - which is against Catholic teaching.
We're all sinners, but the question is whether or not the church or school will accept the "flaunting" of sin as if it had no consequences.
The Church allows divorced people to receive the Sacraments. Remarried people without annulments not living as brother and sister are a different case.