Reprobada means 'failed' or 'flunked', often used in an academic context. It's an interesting word choice in a song about relationships, making it stand out.
In "Amor Express," the singer uses it metaphorically to tell someone that their approach to love is completely wrong, almost as if they've 'failed' at understanding how a serious relationship works. He sings, "Estás reprobada en psicología," implying her romantic strategy is a complete flop.
Banda MS is here to tell us a story about saying no to casual flings! In "Amor Express," the singer is definitely not interested in a quick adventure. He makes it clear that he's looking for something serious, not just fun. He sings, "Tu prefieres lo informal y yo / Yo ando en busca de algo serio" [You prefer informal, and I / I'm looking for something serious].
Our singer is pretty blunt about his feelings, even telling the other person, "Estás reprobada en psicología" [You're failing psychology]! He's not going to be anyone's "payaso en ninguna fiesta" [clown at any party]. He dreams of a "niña bien" [good girl] and won't settle for anything less. So, if you're on his list for a quick romance, he says, "elimíname" [eliminate me]!