A Man for Marcy by Rosamond du Jardin (1954)
Marcy Rhodes and some of her friends face the start of their senior year in high school with a sinking sensation. Last year they had dated only senior boys, who are now off to college, and the girls find themselves high, dry, and desperate.
When a club called "The Widows" is organized, Marcy enthusiastically joins the "mourners," and by the time her erstwhile steady arrives home for Thanksgiving vacation, Marcy is wallowing in self-pity. And her mood doesn't improve when she learns he has been dating at college!
Only the skillful intervention of Marcy's brother Ken averts disaster. Thanks to him, and to Marcy's basic good sense, the balance of the year is gratifyingly different. (from the back cover)