By Sunday School are you referring to the Children’s Liturgy of the Word (which would be children ages 4-7 who have not received first communion), to RCIA for children-the children’s catechumenate (which would involve children over 7 who are not batpized), to CCD/PSR/Religious Education (children in grade 1 or higher, preparing for sacraments or already have first communion), or some type of child care or preschool program?
If you are referring to the first, CLOW, there are several threads on that topic, which discuss that program (permissible) and its abuses (manifold and horrendous). There is no reason the liturgy of the Word for children should take more than the Liturgy of the Word duing Mass. The children should be brought in well before the Eucharistic Prayer beings. The only thing that would make it take longer is some activity that is not part of the LOW–coloring sheet, craft etc. Any handout or activity sheet should be sent home, not completed during this session.
CLOW is exactly what it says, proclamation of the readings for the day from the approved Children’s Lectionary (no other source permissible) and teaching geared to their level of understanding (preferably by a deacon or a priest, or by a qualified, prepared lay person acting under direction of the pastor). It is not a catechetical session, it is not part of RCIA, still less is it babysitting, – it is liturgy, its only purpose is in making this part of the liturgy more accessible to children.
If the new director is making it serve multiple functions, she does not know her job and should be challenged. Under no circumstances (except possibly occasionally when a parent is assisting this program and must keep her children with her) should children who have made first communion be taken out Mass. They are obligated to assist at the full Mass, including proclamation of the Gospel and homily by a priest or deacon.