Good question. "If I WERE a boy" is grammatically correct. Using "were" here is called the "subjunctive mood", which is used to talk about things that aren't real. (The subjunctive is usually formed by taking the third-person-singular verb conjugation and putting it in the past tense, which for the verb "to be" is "were.") In this case, this is a "Second Conditional": Beyonce is talking about an unreal/hypothetical thing in the present. Right now, she isn't a boy. She's imagining what she would do if she were. Some other examples: "If I were rich, I wouldn't need to work" or "If he were American, he wouldn't need a visa to visit the USA." Note that the use of the subjunctive is dying in English. Most modern native English speakers don't use it anymore because it sounds strange. They'll use "was" instead of "were."

conditionals, условное наклонение

because in english when you use "If" , is gramatically different. ex: if He is at the school today, where is He? look for "if" uses :)