Tenth grade dance

"During the middle of a toast to the bride, you don't have to interrupt yourself to tell them you accidentally skipped the part about drinking wineat her house before a tenth grade dance."

What is a tenth grade dance? What does it have to with wedding ceremony? I would be very grateful if tell me what is the meaning of the sentence? Many thanks in advance.

The occasion for this hypothetical speech is not a wedding ceremony, but a wedding reception—the celebration after the ceremony. In the US this typically involves elaborate toasts to the bride and groom by participants in the ceremony. The "best man" and "bridesmaid" are close friends of the bride and groom, and their toasts often include comic reminiscences designed to entertain those present by mildly embarrassing the couple.

Readers of this passage would understand that an allusion to drinking wine at the bride's house before a tenth-grade dance implies just such a reminiscence: it would have occurred some years before the wedding, it would have involved some degree of (presumably amusing) insobriety, and it would have been improper—in most parts of the US it is illegal to serve alcohol to people under 18.