I don't know that anybody can give you a concrete answer. I can say that having a plural and non plural keyword will not outright cause any penalty as our website uses this strategy for some of our pages. These pages have been ranked as far back as 50+ and over the last year have found their way to top 10 results.
In your example the more daming issue would be the repetitive "Wood" in the title then the plural versions. When creating a title tag I find that it is very easy to get caught up in the keywords which is not good, you should focus on clearly and directly describing your page that is after all what the "Title" is for. Further harm can be done by focusing on just a couple keywords as you inadvertently discount long tail keywords, which can be as powerful as keywords you focus on. The best approach is to know your keywords, use them in the title and accurately title your page.
To expand on your example..
Wood Desks - Home Student Office Desk designs by | Furniture Store
This tile has both Wood Desk and Wood Desks as its first 2 keywords, and uses nearly all the remaining characters to include more information about what would be found on this page. I'm also not a fan of ending with Brand / Domain unless it is going to be heavily marketed.
Hope that helps,