"
You're strumming on my heart strings like you were a grade 8 but I never felt this way
Ed is talking about a girl who he is obviously has feelings for. "You're strumming on my heart strings" means that she was flirting with him, or messing with his heart's feelings for her. "Like you were a Grade 8" doesn't mean like she was in eighth grade, which is commonly thought. When ed says "Like you were a Grade 8" he is explaining that she is a very talented musician. In the UK (where Ed is from), music exam grades are given using a scale of 1-8, 8 being the absolute best. Ed is saying that she "was a grade 8", therefore, she was a terrific musician.