From the FAA Glider Flying Handbook, page 6-2 (8083-13A, the current edition):
“The assembly of a glider to include the installation of glider wings and tail surfaces is classified as operations functions not preventative maintenance.
This information can be found in Amendment 43-27, published in 52 FR 17276, May 6, 1987 which is an amendment to 14 CFR part 43.”
The FAA considers that training in assembling a glider is part of the mandatory glider pre-solo training requirements in CFR 61.87(i)(13). Therefore, it not a part 43 preventative maintenance action which would require a logbook entry, but a normal operational action and no logbook entry is required.

Neat!
Back when I first started flying gliders, an instructor told me that you had to log assembly and disassembly in the logbook. Seeing as that was 1986, it was certainly true then!

Here's the link to the exemption, stored at the Library of Congress:
https://www.loc.gov/item/fr052087/

You'll find the exemption listed in Chapter VII, near the end of the document.