Yes, only members of the presidency teach Sharing Time lessons. This was published in the new Church Handbook of Instructions in section 11.4.2 (http://www.lds.org/handbook/handbook-2-administering-the-church/primary?...)

Members of the presidency take turns conducting sharing time. This time normally includes the following elements:

1. Prelude music, a reverent song or hymn that the children know, and an opening prayer by one of the children.
2. One or more of the following: a scripture passage chosen and read by one of the children, memorization of an article of faith, a brief message by a member of the bishopric, one or two activity songs, and children’s talks that are related to the month’s theme.
3. Gospel instruction by the Primary presidency. This portion lasts about 15 minutes. Members of the presidency use the scriptures and follow the current outline for sharing time as they prepare and teach.
4. Singing time conducted by a music leader. This portion lasts about 20 minutes (see the current outline for sharing time).
5. A closing prayer by one of the children, followed by postlude music.