but at the end of Perfect Chemistry, Paco enters the same chemistry class his parents took. Paco Fuentes is the child of Brittney and Alex. Read the epilogue of the first book and you will understand.
This answer contains spoilers…
(view spoiler)[It's not the same Paco. The main characters named their son after him in respect, which is why a kid called Paco shows up in the same chemistry class years later in the epilogue. (hide spoiler)]