A Maryland nurse accomplished two rare feats in her battle with the Internal Revenue Service: She defended herself against the agency's lawyers and won, and she got a ruling that could help tens of thousands of students deduct the cost of an M.B.A. degree on their taxes.

The U.S. Tax Court handed Lori Singleton-Clarke her victory last month, saying the 47-year-old Bryantown, Md., woman had properly deducted nearly $15,000 in business school tuition. The Tax Court ruling should make it easier for many other professionals to...