Um, my college student was just sent home mid-semester on "medical leave" because he did not attend even one class due to anxiety (unclear as to whether the anxiety caused him to miss class or happened after he realized he was pretty much fucked). The school gave us a 100% refund for the semester (incredibly) and he can apply for reinstatement, but they definitely weren't going to let him just finish out the year. (he is a Freshman, BTW, at a major private university).
But back to Parenthood (the fake kind, not the real kind, because I'm tired of that right now)... it would have made more sense to me if Joel left Julia because he was sick of her making major life decisions without bothering to check with him or just ignoring his input (quitting her job, trying to buy the coffee lady's baby, etc.). Otherwise it just feels like a contrived storyline.