I am looking for proof books for my senior sessions. I've been using a local lab, but have decided to use a new online lab. But they want the .jgp quality of 10.

Why? Does the image look bad to the client?

There isn't really a noticeable difference between 10 and 12 and 10 is a smaller file size to work with and doesn't compromise the overall final quality of the image.

10 and 12? So is this something different that then quality slider (1 - 100) next to the jpeg quality slider in Lightroom?

IIRC Lightroom doesn't have the 1-12 scale, only photoshop which they are going off of since it is the mainstream standard.

We have found that in Lightroom, between 80-85 seems to work just fine.

If you work jpgs a lot, open adjust, save, open adjust save, etc, I would work in PSD's then convert to JPG 10 as the last step. We have printing 20x30's at 10,11,12 and tiffs, and you can not see a different in the print quality, even with a loupe.